I don't recall any videocard outside of 9700Pro that was more than 2x faster on average in videogames than a previous generation. HD6990 has 80-90% scaling compared to a single HD6970 (and sometimes nearly 100%). So if you think an HD7970 will be 30% faster than an HD6990, that would mean more than 2x faster than a single HD6970. I could only see that happening in resolutions above 2560x1600 or in heavy Tessellation scenarios assuming HD7970 increases tessellation performance over 6970 by 3-4x. Otherwise, I think a 60-70% performance increase over an HD6970 is more realistic up to 1920x1200 and 70-80% at 2560x1600.
The other part of the equation has to be a larger prevalence of more advanced games. Right now a GTX580/570/HD6970 slice through just about any game outside of Witcher 2 with Ubersampling at 1920x1200. Outside of 2560x1600+ users, I feel that we need a new generation of engines/games to really benefit from another 2x performance boost in GPU speed on single monitors. Even so, how many people will buy a new generation $400-500 GPU just to play BF3? There has to be 5-6 "A" level titles that make HD6970/580 feel slow before there is a real incentive to upgrade.