You may want to take a good look at the XFX Radeon HD 4770.

It's cheap, it plays all gams on maxed out settings (even crysis and Far Cry 2) with the lowest FPS of 30.

If you're packing a 300W to 400W Power Supply, this would also be a good choice as to save on power necessary to run it. It's a 40nm.

Would be nice if you could reveal your computer specs. No biggy though..