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