Often this step is the one which takes the longest amount of time and consideration. Which parts do I buy? Which are best?

There are some great places to buy computer parts, such as Fry's Electronics, Best Buys, or online direct from your favorite computer website. You can find what you are looking for at a computer retail store in your area, however you may end up paying more for your computer. I purchase most of my computer parts from computer shows or from online dealers direct. You can get a better price for a better product when you know what you are building and where to buy the parts.  The best thing about buying retail is that you will pay a little more than you would in other places, but they offer great warrantees.  You may even get lucky and get a "Sales Representative" who actually has some computer building background!

Most towns also have smaller stores that sell and repair computer equipment. Not large retail stores, but  usually  a small office or suite in a strip mall. Regardless of location, though, such stores are often cheaper and can provide individual attention. Their hardware is often retail packaged from the manufacturer, in a box and all. But, they also sell a secondary brand of hardware, usually packaged with very little documentation. You will need to be cautious with  this type of hardware unless you are confident in your abilities to build a computer without detailed instructions. 

The most important thing to remember is to know what type of operating system you want and to know your stuff. The PC sales industry is a successful one, and a very competitive one, and there are many who will take advantage of your lack of computer knowledge just to make that final sale..

Now, let us go through each type of hardware:

Case: Your computer case has multiple functions. It houses all the various components that make up your computer, and it usually comes with the power supply that supplies electricity to your computer system. Don't overlook the importance of the computer case, look for a sturdy, well-built model, that gives you ample room to work in and enough power to suit your needs. Look for a computer case with the power supply on top as this is a more efficient way of keeping heat out of the case. Choose a sturdy case and make sure that you don't compromise the interior for the exterior.

  Motherboard: Most everyone knows that the motherboard is the most important component of your computer. Every other component, at some point, connects to the motherboard. Also, the motherboard controls what you can and cannot put into your computer later on down the road. Want to upgrade you RAM? Well, you first have to check to see what types your motherboard will take, and whether it can support that much memory. Want that new TNT2 video card? Well, does your motherboard have AGP? Get the point? If you choose the wrong motherboard in the beginning, you may find yourself having to buy another down the road to support some other upgrade. Today's motherboards are a lot different than the one's in the 486 days. If you are used to these older systems, you need to get up to speed with the latest boards. Where you needed an IDE controller card the connectors are now built right on. Where USB was optional, now every board has it. Some boards go all the way, offering built on SCSI controllers, 10/100 Ethernet support, onboard video and sound, etc. Buying a motherboard is a trade-off. You need to know what you want and then pick that board which has the best combination of gizmos for you.

  Three Levels:

There are really three levels of motherboards. Of course this is a loose look at the market, but its accurate enough.

First, we have the cheaper, bare-bones boards. These are the types of boards you usually get if you are not into PC hardware and don't want to deal with frustrations. You just want to build it and turn it on. These boards have built on sound and video, usually, and sometimes other gizmos too, like a modem. They don't usually overclock well and don't have a wide range of CPU support. These boards are cheap.

Second, we have the level of board that most use, like the Abit BX6 or some of the newer Tyan boards. These boards come with a single CPU slot, EIDE controller, etc. Most don't have built on video and sound. They support a wide range of processors, and with more voltage and multiplier settings, they are more overclocking friendly. With the proper amount of PCI slots, these boards are great.

Then you have the beasts which most of us cannot afford. These are the dual processor boards, often with built on NIC and SCSI, a crapload of PCI slots. These are more for NT workstations than your desktop PC.

Board Layout:
A lot of people don't consider where everything is placed on the motherboard, but it is important. Is there a big capacitor right near the CPU slot, blocking where your CPU fan will go? Is there a bunch of crap that will block your full-length PCI card from fitting? You need to know roughly what you will be plugging into this board and know if anything will get in the way.

PCI Slots:
If we had our way, we'd have a motherboard with 20 PCI slots so we can run everything in the world. But, this doesn't exist. So, you need to pay attention to how many PCI slots a motherboard has. For most the standard 4 or 5 PCI slots will be fine. But, if you want to stick some nice stuff in, you'll soon run out. Look at all the stuff you can get? NICs, dual VooDoo cards in SLI mode, SCSI controller, MPEG decoder. You can easily fill your slots. I had a Asus P2B and ran out and got a P3B-F just because it had an extra PCI slot.

Manuals:
Believe me, you'll regret it if you don't think ahead and get a board with a good manual. If you buy a no-name board, you'll probably get stuck with a manual that was written by someone in Taiwan who doesn't know English. A lot of times, you'll find a pile of addendums added to the manual. They couldn't get it right the first time?! Make sure to look at the manual for your board and make sure you can understand it. Most well-known brands have decent manuals. Asus, Abit, FIC, Tyan, and other good ones come with nice manuals. Another thing to keep in mind is that the better known manufacturers often have nice web sites, and you can sometimes get support info there too.

  Form Factor:
Unless you would die without that older server case you are using, I would recommend going with the ATX form factor. ATX integrates all of the connectors, whereas with AT, you have to plug all of that crap in. AT boards use that big DIN keyboard connector which is a relic. You can check out the differences between AT and ATX here.

  Chipset:
The chipset is the hub of your motherboard. You need to pay strong attention to what chipset a motherboard has before purchase. The chipset is fully responsible for what hardware your motherboard will support now and in the future. It controls everything. If your motherboard won't support ATA-66, AGP 4x, Direct RAMBUS, etc. blame the chipset.

The times are moving quickly beyond the BX chipset. With the advent of newer technologies, the Intel i820 chipset is a biggie to look at. Via's Apollo Pro chipset is the top dog for the Super 7 systems. And, of course, if you want to build an Athlon based system, you need a Slot A chipset, like AMD's own 750 chipset. The Intel BX is still a powerhouse, though. It supports a lot of features and is a very compatible chipset, but to take advantage of some of these newer items, you'll need to move on.

Hardware Support:
This is a function of the chipset, really, but you need to look into this. First, you need to know what processors this thing will support. Many Celeron boards won't support the Pentium II or Pentium III, for example. And don't expect a Slot 1 board to run an AMD K7 Athlon. To get an idea on the upgradability down the road of the processor, look at the board's available multipliers. For example, you may want to run a Pentium III 450 at a 4.5x100MHz setting. But, what if you want to later be able to upgrade to a 650MHz? You want to be sure this board will support that 6.5x multiplier.

PC100 memory support is almost standard now. But, we now have 133 MHz SDRAM on the market, and you want to be sure your motherboard will support this, if you're interested. Also, does it support Error Correction, or ECC, memory?

Another big thing most people forget about: disabling of on-board components. You may think its the greatest thing since sliced bread that your motherboard comes with integrated sound and video. But, what if you later find that your on-board video is a relic from the ice age and you want to plug in the latest and greatest video card on the market and play some games? Well, you need to be able to disable the on-board circuitry first. Many boards don't let you do this, or some only with a lot of name calling and slamming around. Be sure any board you buy with these features also gives you the ability to easily disable it.

Reviews:
Finally, before purchasing any motherboard, find out what others think of it. You can rest assured that if there is any nagging annoyance with any motherboard you are thinking about buying that several people have noticed it and posted crap all over the internet about it. Check out hardware review sites. I'm sure you all know where to go for those. Check out Usenet Newsgroups.

Processor : The CPU (central processing unit) is undoubtedly the most important part of your computer system.. It performs the myriad of calculations necessary for the computer to make its decisions. The CPU does not have one standard size and interface, so make sure the Motherboard you purchase is compatible with your CPU. After years of inovations their are currently only two major chip manufacturers, INTEL and AMD, with INTEL being the overwhelming market leader. There are four processor families on the market today. The Celeron and Pentium processors from Intel and the Athlon and Duron models from AMD.Processors come in three basic levels:

Celeron

 Intel introduced this chip as a low-cost high performance alternative to the high end Pentium III processor. Demanding "Low End" consumers as well as increased low-end competition from rival chip maker AMD has forced INTEL to continually raise the performance bar on the Celeron giving it almost Pentium III like performance at a considerably lower price. The main difference between the Celeron and The Pentium III is the amount of High Speed L2 cache memory the chip has access to. The Celeron generally has half the amount of memory resulting in reduced performance in some applications, but to the average user, this is barely visible. Another thing that slows the Celeron down is its Bus speed. The bus is a pipeline that sends information back and forth between the processor and other computer components. A faster bus translates into faster processing and computing. 

Duron

The Duron is AMD's entry into the high performance, low cost, chip market. AMD and Intel are in a constant battle of trying to one up each other, both in the high and low end of the market. With the Duron, AMD once again captures the performance for a fraction of the cost. The biggest difference between Duron and Celeoron is in the way it handles information, allowing it to come and go more quickly, and allowing for significantly improved performance. While the Celeron crawls along at 66MHz bus speeds the Duron operates off a 200MHz bus. This contributes significantly to overall system performance. Clock Speed on the Duron ranges from 600MHz all the way up to 800MHz The Duron uses a Socket A format to connect to the Motherboard.