Well, the only high end models in the GTX 600 that are on the market at this time are the 670, 680, and 690. The 670 is, at most, about 7% slower than the 680 (usually more like 3% to 4%), so it is what I would recommend, especially since some 670s seem to support quad SLI and they are so much cheaper than the 680, in addition to using a little less power.
What do you want to do that would need two 670s or 680s in SLI? It's a heck of a lot of GPU performance for gaming at anything below 6MP resolutions (triple 1080p and similarly intensive configurations) for even the most intensive games.