Caporegime
One thing I have picked up from having a good play with both is that a faster card does not equal a smoother gaming experience. For me the 7970 is a faster card in just about everything I have tried but it does not give as smooth a gaming experience as the 680 which on paper is a slower card.
Nvidia cards do not offer good value for money and if buying now AMD is the way to go as nothing comes close to the value of a 7950. But if you are wanting the best all round gaming experience then a 680 is the way to go. I have not played with 670's so cannot comment on how they compare to a 680.
Good info Moogleys and for me, I would rather pay more for a smoother experience. It is good to read users who have used both and see their feedback.