Here's my recommendation (minus case/power supply & CD/DVD drive) for a decent gaming machine with plenty of room for expandability:
http://secure.newegg.com/WishList/Pu...Number=8765906
All of that for the low, low price of $510.44 + shipping. The best part is that you can go SLI down the line for another $100, the mobo supports AM2+ so you can upgrade to a quad core down the line, and you can still shove another 4GB (for a total of 8GB) of RAM on the mobo. Just find a DVD drive, a case/power supply, and that should get you where you want to be.
On a side note, if you're looking for specs on video cards, I recommend these pages on Wikipedia:
Comparison of Nvidia graphics processing units
Comparison of ATI graphics processing units
Hope this helps
