I'd do a 670 myself .... but if ya wanna compare.....this is the widest series of tests I have seen (and by that I mean various settings and resolutions) here:
http://uk.hardware.info/reviews/2725/8/nvidia-geforce-gtx-670-review-plus-sli-and-3-way-sli-aliens-versus-predator
Using these games .... AvP High 4AA, Batman AC Very High 4AA, BF3 Ultra High 4AA, Crysis 2 Ultra High Edge AA, Dirt 3 Ultra 4AA, Hard Reset Ultra 4AA, Metro 2033 Very High, Skyrim High 4AA and Shogun 2 Ultra 4AA, we see:
The 670 gets 650.2 fps total in those games at a cost of $0.54 per frame
The 7950 gets 530.4 fps total in those games at a cost of $0.57 per frame
The 7970 gets 629.8 fps total in those games at a cost of $0.64 per frame
The 7970 GHz gets 732.2 fps total in those games at a cost of $0.61 per frame
The 680 gets 711.2 fps total in those games at a cost of $0.66 per frame
Costs are based on lowest price from major vendor (EVGA, Asus, Gigabyte, XFX)
The 7970 is great buy with the current $30 rebate from XFX which knocks the above cost per frame down a bit ... you'll have to weigh that against the Boredlands 2 coupon that comes with the MSI 680's
However, at newegg's current price of $420 for the 7970 Ghz, you'd have to put that high on the list. The Ghz is already highly overclocked so you might want to compare it with some factory overclocked cards from nVidia.
At Techpowerup .....(Alan Wake, AvP, Batman Arkham City, BF3, BattleForge, COD4, Civilization 5, Crysis. Crysis 2, Dirt 3, Dragon Age II, Hard Reset, Metro 2033, Stalker CoP, Starcraft 2, Shogun 2, Skyrim, WoW,)....
The 7970 GHz gets 1763.2 fps total in those games at a cost of $0.24 per frame ($420)
The 670 DCII gets 1770.8 fps total in those games at a cost of $0.25 per frame ($440)
All in all, Id say it's hard to go wrong with either one though what each card does in its hardest challenges may be worth looking at. there are other things to look at such as frame latency and CPU loading effects
http://www.tomshardware.com/reviews/fx-8350-core-i7-3770k-gaming-bottleneck,3407-9.html
Our benchmark results have long shown that ATI's graphics architectures are more dependent on a strong processor than Nvidia's.
You have a strong CPU so I wouldn't worry about this issue.
As far as frame latency, if one card wins 150 fps to 120 fps do you care ? On the other hand, what if both cards average 50 fps in their biggest challenge but one sometimes drops to 25 and the other drops to 35 at times ?
Ya may want to investigate how this issue might effect the higher end cards. I haven't seen anything as of yet. The article below is one of the few to addresses frame latency but as it compares the 7950 and 660Ti, hard to say how it scales up to the top end cards.
http://techreport.com/review/23981/radeon-hd-7950-vs-geforce-gtx-660-ti-revisited/11