this is kind of funny, because toms did an article on a similar question literally yesterday.http://www.tomshardware.com/reviews/geforce-gtx-680-geforce-gtx-660-ti-sli,3429.html
Basically the TL;DR is that two 660's are the best value so long as you don't mind SLI and the issues that come with it. I've always had more powerful single card setups.
I read the article right after this post. After reading it, I was planning on doing two 660 Ti's (maybe 3 down the line somewhere if possible).
But I'm glad I made this thread as others point out some definite benefits of a single card over multiple. I like the driver compatibility and lower heat issues. I think I've been swayed towards a 680.