Two GPUs actually can give an 80-90%+ performance boost in games...
Anyways, if you want to overclock your graphics card, get a nice custom 7970. Overclocked, it will match overclocked 680 performance and thoroughly beat a 670.
If you want to just plug in your graphics card and not worry about it (this sounds like you based on your posts), then get a 680 if you can find one at a decent price.
If you want the best deal for your money at the high end of the spectrum, get a 670. It gives you close to 7970/680 performance for $50-100 less money.
It all depends on what your desires are. Things are close enough this round that you cannot simply say "X card is better than Y card" without further qualification.