CPU - Athlon II X4 3.0 (3.4 O.C.)
Mobo - ASUS M4A88TD-V EVO
Ram - G.SKILL 4GB (2 x 2GB) DDR3 1333
PSU - Rosewill Green Series RG530-S12 530W
Monitor - Acer G205HV 1600x900 max
Based on your system specs (and predominantly your resolution of 1600x900) I'd go with an ATI 5770 (or ATI 5850 just cause they are about the same price right now). For nVidia's side of things a GTX 460 (even 768MB) would be plenty of power. Anything more than the ATI 5770 (even the GTX 460 or ATI 5850) is more than you need for most games.
Now, if as others have brought up, you want to max out Crysis or Metro 2033 (which are absolute beasts) then you could consider something more impressive (GTX 470, ATI 6870, etc). But that's just two games that don't perform well, but look nice.