Ok, I'm doing all my pricing for GPUs and I'm trying to decide between one large or 2 smaller from a previous gen. 

I've looked at GTX 770 vs  2 GTX 660Ti but I'll look at anything comparable

one factor to consider, do you have sufficient power to drive two gpu as well as the other components whilst keeping it cool.

Do you already have an old 660TI?

Screen size and resolution?

Single card solutions are always more consistent. They've fixed most SLI problems but you're still going to have larger peaks and valleys with two cards resulting in a more uneven experience simply because the cards have to communicate over a small SLI bridge.

IMO 770 or a 280x/290 non-reference editions.

Its for a future build, the power estimation is 440w with a 750w supply, that with the 770 in it. But if SLI has consistency issues I'm not going to worry about it.