+1 to crisan on the GTX670 recommendation and er... -1 to unksol. The 7970 does indeed get more frames/sec on average, but by 5% or less (more in AMD-friendly games, less in nVidia-friendly games of course). Even at 5%, that's the difference between 30fps and 31.5fps.
The 7870 is a bad recommendation because you won't be seeing a huge gain over what you have. Obviously it depends on what you consider a worthwhile upgrade, but I'd personally be shooting for a 50% gain (enough to turn 25fps into 37fps). For that kind of improvement, GTX670 is definitely the way to go.