Newegg.com is an excellent site to buy computer parts. Very good prices.
pomx72, I'll gladly guide you through the process of getting a new computer. Here are some basics.
Microprocessor (CPU)
The main players right now are AMD's Athlon64 X2 line and Intel's new Core 2 Duo line (nicknamed Conroe). It seems Intel's new line is a better choice.
Motherboard (Mobo)
The principal and most important feature of a motherboard is the socket it has. This is what defines what CPUs can be mounted on the board.
In this category, you have many options. If you chose an Athlon64 X2, then you'll want to get a motherboard with socket AM2 (socket 939 might work too, but it's becoming outdated). If you chose a Core 2 Duo instead, you'll need socket LGA775, sometimes called socket T. Be careful here, because not all LGA775 boards are compatible with Core 2 Duo; always check in the board's specs.
The good brands in this department are DFI, Asus, Intel, MSI, and Gigabyte, though Asrock, Abit and ECS sometimes offer competitive motherboards.
Memory (RAM)
You'll want 1GB or more of RAM. Many people are shooting for 2GB right now, but 1GB is enough for most applications.
The newest type of memory is called DDR2 and it's what you should aim for. DDR2 comes in different speeds, mainly 533Mhz, 667Mhz and 800Mhz.
The recommended memory brands are: OCZ, gSkill, Crucial, Corsair, Kingston, Mushkin and Patriot.
Hard Disk Drive (HDD)
Here you'll want to focus your attention on two excellent brands: Seagate and Western Digital. The relatively-new SATA interface is preferable (this is how the HDD is connected to the motherboard; just look for "SATA"). Apart from this, there's no much difference between hard drives except their capacity in GB. Just get what you need.
Video Card (a must if you like to play games)
Again, there are two big families you can chose from. First, there's the nVidia Geforce 7 family, which include the following models: 7300, 7600, 7800, 7900 and 7950. Second, you have ATI's Radeon X1k family, with the following options: X1300, X1600, X1800, X1900 and X1950.
There'll also be different version of each card model, usually indicated by two or three letter after the model number: 7600GT, X1800XT, and so on.
You should know that nVidia and ATI don't make the cards themselves; they only make the GPU (the graphics chip). What I've posted above are GPUs. Other companies are in charge of making the actual video card. That's why you'll see things like "XFX 7600GT" and "eVGA 7600GT". Here 7600GT is the model of the GPU (made by nVidia) and XFX and eVGA are "manufacturers" who make and sell the card itself.
Good manufacturers are: eVGA, BFG, XFX, Gigabyte, Sapphire, Powercolor, MSI, Asus, HIS, Chaintech and Leadtek.
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That's all for now. We're still missing some parts, like the Case, the Power Supply, an Optical drive, and of course Keyboard, Mouse and Monitor.
Any questions so far?