I've had the GTX670, 680 and now HD7950.
The 7950 is factory o/c to 925MHz and was equal to a completely stock 670 in both 3dmark11 and Heaven11 benchmarks.
The 7950 o/c well to 1200/5000 or 1100/6800 where it then beats the GTX680 o/c to 1032/6000.
The 680 has to be o/c to 1150MHz to then pulll ahead of the 7950.
The 680 needed 1.076v to reach 1032/6000
The HD7950 needed 1.081v to reach 1000/6000 (factory o/c was 925/5000 with 1.25v). The AMD card has a 384bit memory bus so the bandwidth is huge at 6000MHz, the Nvidia card only has 256bit.
In reality, a lot of it depends on the games and drivers. eg. a single 680 was awesome in Crysis 3, yet 2x 7950 in crossfire only just beat it. And this is an AMD sponsored/optimised game, but clearly worked better with an Nvidia card.