So would it be a problem if I wanted to SLI GTX 660's in the future?
Not really. I was talking more about SLI and Crossfire with the top end cards, as well. Something like 2x+ SLI 690's, or 3x+ SLI/CF for 680's and 7970's. Meaning that's what it would take for 2.0 to start being a bigger problem.
2 660's shouldn't be bottlenecked much. Again, not strong enough to be, really.