That chart you posted is great for a point I want to make before we get into any more HD7970 vs. GTX680 debates: Whatever performance delta existed among GTX275/4890/GTX285 at the time just about evaporated later (i.e., if HD4890 became too slow in modern games, so did GTX285). I'll present my case that essentially when GTX285 cost $350 and HD4890 cost $269 the $90 extra in future-proofing didn't really help.
The same can be applied to today. If your GTX480 is choking in games, GTX580 is unlikely to alleviate the performance issues you are having. So in 2 years from now the 5-15% performance advantage GTX680 might have will evaporate into "Guys, I want the shiny new GTX780/HD8790, etc." Basically the whole point with "future-proofing" of 3GB of VRAM vs. 2GB might not matter that much since in 18 months $500 GPU users will have migrated to the next generation.
Every generation we end up splitting hairs over 15% performance differences and then deem that entire generation too slow when we upgrade.
The real "GPU crusher" AKA GK110 isn't here but at least we are seeing some price drops, even if it's $50.
Do we get official benchmarks tomorrow?