It's not official yet, but retailers have reported the SRP of the GTX 680 has been dropped to $499. I've seen dozens upon dozens of benchmarkings and tests playing actual games that have shown the GTX 680 to be consistently faster than the 7970, though some of those tests were questionable, some seemed fairly straight up. Also of note: the GTX 680 has a TDP of only 195W, while the 7970 has a max of 230W.
It's thought Nvidia may have lowered the price to start another price war with AMD, which, looking at their profit margins lately, can't really afford to drop the price of the 7970, though they may have to anyway. Happy shopping