7950 looks like the best bang for your buck in that price range. I think the 580 does beat the 7950 in a few benchmarks, but vice versa is true. It's a hard call to make, but I will be getting the 7950 due to the cheaper price. Aparently I will be missing something called PhysicX on the geforce cards though, but I still need to research what this actually does.
Have a look at the benchmarks yourself, you may be able to the score against the particular games you will be wanting to play:
http://www.youtube.com/watch?v=q57cuLC9zLw
"star" me if this was helpfull