Agree w/Forceman ... the 670 is much better value than the 680. Depending on the individual sample you get, after overclocking, some 670's are faster than some 680's. For example, I'd bet that my particular 670 (which will do about 1365c/7100m) is faster than probably 25% of all the 680's out there ... talking w/both cards OC'd to their max clocks. And no 670's are ENOUGH slower than any 680 that you'd actually 'notice' the difference in the gaming experience with the naked eye.
SLI is fun and everything but honestly a high-end single card is a better option than SLI'ing two mid-range cards, even if the SLI setup has a better FPS/$ ratio. And this is coming from someone who ran Crossfire or SLI for the better part of the past 4 years.
Because of the effect of microstutter, two cards in SLI need to run at quite a bit higher FPS to match the smoothness of a single card. Like, 45fps in SLI is about as smooth-feeling as 35fps in a single-card. Once you take that into consideration, the 'value' of choosing the SLI setup goes down noticeably.
On top of that, you have the extra heat and noise concerns, + the possibility that you encounter games where nV has not yet made an SLI profile for the game. When that happens (unless you hack yourself a solution), you'll have the performance of only one of your two cards.
If I were you I'd choose a 670