My budget is 400-600. I'm contemplating one GTX 680, or two 660s.
The 680 would be the better choice in the long run as I can get a second one cheaper in a year or so. But I'm wondering if there is a huge performance gap between two 660's and a single 680? If so, maybe I go SLI now and ride that out for a few years.
go for a single 680
with a single gpu you will experience less driver issues meaning a new game comes out and your two video cards are not being utilized the best because of either new drivers or the game is not optimized for that yet. also two cards will run louder and hotter than a single one
Always go for the single gpu option the only time I would recommend SLI/Xfire is if you get the 2 highest end single gpu's for that generation or if you have say 1 gtx 660 already and you struggle to run games in like a year or so then upgrade and it would be the most cost efficient
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.