A 4gb 670 is probably best for 3d as the extra 4gb will help with 3d. And a 2nd 660 ti will help, but it would be hard to use with 3d because you would still have only 2gb of vram, which isn't good for 3d gaming.
I tried out the gtx 670 and was able to push 3 asus VG248QE at around 5700x1080 in 3d but i had to drop to medium settings with most games with the asus directcu 4gb version. If you are only going to push one monitor and it was 1920x1080 i'd say a single gtx 670 would be a better choice than running 660ti sli. The smaller memory bus is something i'd be wary about on the 660ti though you might not have any issues. Also the fact that you would be running a single card as compared to 2 is much better. Don't bother getting the 4gb though as I never found any game that would use that much on a single monitor. It wasn't until i went up to 3 monitors that it started working for its money. So my vote is a 670.