7970 is actually on par or faster than a 680 on most games with 12.11 drivers. Of course, there remain some games which prefer Nvidia cards, but BF3 which was a major win for them now runs better on AMD cards.
I'd also be inclined to say Crysis would likely be optimised better for Nvidia but I've just checked a few Crysis/Warhead/2 reviews, the first two seem to prefer the 7970 with extreme settings, while Crysis 2 Ultra marginally preferred the 680 prior to 12.11 but the opposite seems to be true now, but there really isn't much in it from what I can see.
I can't find any games which run faster on 670.