You have a nice rig for gaming, a good processor, good PSU, but it lacks a bit of ram, perhaps because you´re are running it with a 32 bit OS.
Anyway at that price range you really can get a cool card, so I will list 2, one from Radeon an another from Nvidia:
GIGABYTE GV-R687OC-1GD Radeon HD 6870 1GB 256-bit GDDR5
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125357
EVGA SuperClocked 01G-P3-1461-KR GeForce GTX 560 (Fermi) 1GB 256-bit GDDR5 http://www.newegg.com/Product/Product.aspx?Item=N82E16814130661