Since you don't have a lot of experience building PCs I have to recommend a Dell. Their new Vostro series is really cheap. You can get one with a Core 2 Duo E7200 2.5 GHz, 3 GB of ram, 250 GB hard drive and DVD burner for $400. Then buy a GeForce 8800 GT from NewEgg and you'll be at $520 total for an absolutely blistering PC except without all of the hassle of building your own.
http://www.dell.com/content/products...740&lid=678195
Start there to begin customizing the Vostro 200 if you're interested (you will have to delete the monitor from the system to get the price down).