I'd recommend Radeon HD 5770 for low end gaming Nvidia GTX 460 for mid-high end gaming and a 5850 for high end gaming.
Lol wut? HD57xx series are mid-end cards, GTX460 is high end.
Anyway, on topic.
Get the GTX460 and OC your CPU. OC'ing the CPU will remove the bottleneck it causes.