Tweaktown Review Comparison
Palit GTX 680 SLI @ 1150-1200 Mhz max Turbo / 1650 Mhz GDDR5 Memory - Link
MSI Lightning 7970 Crossfire @ 1200 Mhz / 1500 Mhz GDDR5 Memory - Link
NVIDIA : 100.98 % Performance
AMD : 100% Performance
Best for Each Removed - (AVP / MAFIA II)
NVIDIA : 98.63% Performance
AMD : 100.00% Performance
It seems like Performance is within error across these tests. The 7970 has a slight, average 50 Mhz, clock advantage, while the GTX 680 has a ~150 Mhz (600 Mhz ) memory clock advantage, otherwise the setups are identical.
EDIT: It is interesting to see that DIRT 3 performance goes to AMD, and Metro 2033 performance goes to Nvidia, the opposite of what is found with only a single card comparison between the GTX 680 and the 7970.