"The GTX 680 is set to show a 45 percent performance increase over the HD 7970, while the GTX 670 shows a 20 percent improvement... " speculative numbers anyway.
AMD pulled quite a marketing scheme by releasing high end cards with extremely conservative clock/memory speeds, that has "High Overclock Potential 25-30%." It sells well to do it like this. You know, release a card with "great potential," then jack up the price so people think they got they're moneys worth. If they wanted they could have easily released these cards with that same overclock back in January.
Maybe by the time Nvidia releases it's 680, AMD will have done a lil bit of tweaking, and will release overclocked cards that will negate that 20-45% improvement, and bring it down to 0-20%. Oh, 7970 drivers should be optimized by then which will bring that performance down even more.
I'm not dissing AMD, well, maybe a lil bit.