Ran a 7970 with a Phenom II x6 at 4.0 GHz for a couple months... no bottleneck in most games, but from my game library, I had bottlenecks in:
Starcraft 2 and all other RTS games
The Witcher 2
Crysis and Crysis Warhead
All those titles saw sizeable performance increases when I went to my current CPU. Some older games like Portal 2 and Dragon Age also saw performance increases, but they already run so well on a Phenom 2 that it's hard to call it a bottleneck.
this reply right here slightly less in actual CPU and GPU strength (though ratio wise might equal out in the comparison)..
I have a GTX 580 SuperClocked and felt the same issues.
965BE @ 4.0GHz and 975BE @ 4.2GHz - including SSD.
and I still went 2500K..