Alright, I was in the same situation as you a few months ago.
Here is a really cheap system that will be pretty sweet
1 GB Dual Channel ROM (Go with Dual Channel)- $70
AMD Athlon 64 3200+ - $160
Geforce - $120
(http://www.newegg.com/Product/Product.a ... 6814130220
Intel Motherboard - $120
Thats a solid $350 right there.
I assume you have an old computer. Take the CD Drive (unless you want to burn CD's/DVD's and your old one isnt a burner), case, and cords from your old computer. Also, you could take the old power supply and hard drive if you want to save a little more. However, it's good to upgrade a hard drive every few years.
Samsung 250 GB hard drive - $120
A 500W power supply will run you about $40.
Thats a pretty sweet $510 dollar computer.
250 Gig hard drive, an Athlon 64 with an average Geforce card and dual channel 1 GB memory will give you a gaming machine able to play anything you throw at it.