Hokay, in response to Drogan and actually building his system.
If you want to take a stab at it, here's a general step by step guide. But as with all computer building, specific boards and such will look different to a greater or less degree.
http://lifehacker.com/515...-a-computer-from-scratch
I'm sure there are other guides that may or may not be better, but this one isn't bad and isn't too out of date.
Generally speaking, you need a Case, a motherboard, a CPU, a graphics card, RAM, hard drive, CD/DVD drive, and power supply for the basic box. Make no mistake, the Motherboard is probably the biggest decision, as it dictates how everything else slots in and what components will fit. If you're buying high end components, you may want to look into some aftermarket cooling solutions (CPU/Case fans)
Generally speaking, you'll want to buy your stuff from the same place unless you find a super deal somewhere. Newegg and TigerDirect are both reasonable. Though there are of course, many other sites.
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If you're going for a complete system with no input from you then, try
http://www.newegg.com/Sto...ory=10&name=Desktop-PCs, or similar sites if you don't want to go straight to a manufacturer. If nothing else, you can go off the specs therein and investigate the different companies. Do be careful about what each system contains, some include the monitors, keyboards, mouse, etc, and others don't, and of course pay attention to what OS comes with it.
For something in between the two you can try custom build sites like Alienware (thought that's probably out of your price-range) or something like
http://www.cyberpowerpc.com/ or
http://www.ibuypower.com/. I'm afraid I can tell you how good those sites are as I don't use them. However, I would look up the components they're using in their systems and see how much of a premium they're charging for putting it together for you.
Whichever way you go, I _highly_ recommend that you do research on each type of component and how much the current/last gen stuff costs and how they perform. Tom's hardware is a reasonable resource for that.
Hope that helps.