Heya mike;
I gotta tell ya i had a hard time getting myself to respond to this post, i just hate building or watching people build substandard computers, especially when they are going to use it for gaming, which is one of the most demanding tasks you can ask your computer to perform.
With this in mind you need to save up a bit more money and buy something you will really be happy with, i suggest a budget of $800.00 US, i know its more then you wanted to spend but it will be worth the extra 300 when you see the performance you get out of it.
You cant build a solid foundation on sand,(well except in this case of course heheh), anyway so we want to start out with a solid foundation, so lets talk about motherboard.
Asus is the best there is, with that in mind spend $200 and get a *Deluxe model of a p4g8x(i7205), or a P4p800(i875) motherboard, these bad boys are awesome.
As far as CPU goes buy a CPU with the fastest Front Side Bus you can afford, i would suggest the, 2.4Ghz p4~800c variant, you can get one on
Http://www.pricewatch.com for $176.00 shipped, both motherboards listed above will run this CPU, and support Extreme Overclocking.
Next lets talk about DDR, you will want to get yourself some High Quality modules that will time at 2-3-3-6, I would suggest you visit this website for memory
Http://www.buyaib.com ,I have some modules from them and they will actually run stable at 2-2-2-6 , while at the same time OCing the external CPU frequency by 15x4Mhz, they have an excellent product, choose Option-C so you get the Cas-2.0 modules.
This will run you $200 for 2x512MB of Micron or samsung pc3200.
As far as video card goes i would suggest the Asus v9280S, this thing is a Monster in performance and value.
It will run you $175 on pricewatch.
I hope this helps.
XeeN