I'd go with 700 or 750 to be very safe.
Also, keep something in mind... the 660s are only better in games that support SLI. Most of the games that are badly optimized and thus have bad performance? Aren't going to support SLI. The general consensus is to not ever sli low-end cards, because when you can't use them, you're running on a low end card.
Yes. They have better oomph. But graphics cards are about SO much more than raw oomph.
GTX 670 is so much money, losing at benchmarks bf3, skyrm, max Payne Ect all support SLI and are double the fps nearly with these gtx 660s. Nvidia I think now are really good adding support for SLI in games.