I suspect that you will win more benchmarks with a GTX660 sli solution than with a single GTX670 FTW.
But, since some games do not play well with dual cards, I think your gaming experience will be better with one good card.
A GTX670FTW will also be less expensive and you will not need a sli capable motherboard or a stronger psu.
If you are running on a single 1080P monitor, you will be very satisfied with a single card.
If you want to spend all of your sli budget, you can do slightly better with a GTX680 and even get back some change.
Don't know if the small difference in performance with a GTX680 is worth it though.
I would go with a 670. You could go with 2 660's and spend around the same amount as a top tiered 670.
I feel you create more issues then solve them with SLi unless you are running mutiple screens in a surround setup 3+ monitors I think its safe to say the 670 would be the better option. And typically rule of thumb its better to have a awesome card rather then 2 budget cards ganging up together to amount to the power of the amazing card.
What looks good in benchmarks does not always translate into real world performance. SO in short I'm in agreeance with geo.
Less Power, Higher level of consistency, Higher Reliability, slightly slower performance in comparison to the 680
there are many ways of measuring performance. too many people go solely by how many frames per second cards put out without taking anything else under consideration. buy the 670. you'll be better off in the long run.