Till you overclock the GTX 680....
Yes but the difference won't be much considering the price premium the 680 commands. If you look at the benchmarks I linked, HD7970 GE overclocked is barely faster than an HD7950 OC. GTX680 OC is not faster than HD7970GE overclocked (in fact it's slower). At best you'd be looking at GTX680 OC at 1300mhz outperforming an HD7950 @ 1200mhz by 10% on average, and it'll cost you a whopping $180 more to get there. There is no videocard out right now in the high-end space that can match HD7950's overclocking and bang for the buck at $280, especially not with what look like to be 2 free solid games like C3 and Bioshock Infinite.
Regardless, GTX680 is at $450 doesn't make any sense since the Asus Matrix Platinum 7970 @ 1300mhz+ would smoke it for only $50 more [hardwarecanucks.com]
. And if you want to SLI, you can save $200 and just brag 2x 670s.