I will be buying a new gaming system soon including a 2560X1600 display. It will have an I5 3570K processor moderately overclocked.
Though an Nvidia fanboy, $600 for two 7950's and $700 for two 670's makes the 670's painful. Especially when the 670's are 2G and the 7950's are 3G and it seems 2560X1600 is were you start wanting more than 2GB. Can any Nvidia fans pull me back? Can any AMD boys pull me all the way over to their side?
SLI 670's will outperform the CFX 7950's until you run out of VRAM, but that's what you get for the extra $160 that would be spent on the 670's. You shouldn't really have a problem with the VRAM limit thing with today's games, but I cannot really say anything for sure about the games of tomorrow. I'd personally go for the CFX 7950's or even 7970's if you want something equal to SLI 670's in performance and price.