GeForce GTX 660 Ti 2 GB in 2-Way and 3-Way SLI Configurations
There's some interesting results there to show how putting two or three midrange cards does against the higher end. But it basically boils down to that the GTX 660 Ti in 2-Way SLI performs better, sometimes up to 50% better against a single GTX 680 for about 32% the cost. 3-Way SLI against a GTX 690 is slightly better, but paying that much for graphics (and real estate in the case) is just WTF. In regards to power consumption though, idle there is barely any difference, but on load, 2-way SLI eats up 130W more than a single GTX 680 and is about the same as the GTX 690. 3-Way SLI is 240W more than the GTX 680 and about 100W more than the GTX 690.
So if you're willing to put up with the higher power consumption and don't mind spending about $660, I think a GTX 660 Ti setup in 2-Way SLI makes sense.
Before you go "but microstuttering!", the perception of it is only obnoxiously obvious at lower frame rates (25 or less) and it really depends on how sensitive you are to it. I have a setup of the GTX 560 Ti in 2-Way SLI and I don't really notice anything.