If you want close to maxed settings on the newer games, get the radeon 7850. If you are ok with high settings, radeon 6870. Don't care about graphics and wanna game on med settings? Radeon 6850.
Right now I'd say the 7850 is the better deal for your dollar. If you are a huge nvidia fanboy the 650 ti isn't a terrible option at $150, but the amd options are more bang for your buck.