As far as frame rates, dual GTX 560ti's in SLI would be way faster than a single GTX 670. But SLI is reported to have some issues with some games. I have 2 GTX570s and haven't experienced any of those issues, but that's what I am told.
Keep in mind though, that boards with the 2nd slot @ X4 bandwidth will cripple the card in the 1st slot and drag down the frame rate big time. I speak from experience.