If you can afford it why would you go for anything less than the 680 sli or 690?
Seems like a silly question. Why? Get the most you can afford!
Quick Edit: Maybe 3 way 670. Still would be with either a 680 x 2 or 690 tho. I prefer the fewest cards possible.
My first problem is the 690. I don't like the idea of having 4Gb shared between the two chips. Higher resolutions need more memory especially if I'm turning AA and MXAA/FXAA up.
The 680's are essentially 670's with a few more cuda cores. Everything I've read seems to indicate that overclocking a 670 will produce an equal or better card to the 680.
I'm leaning towards a pair of 670's or a pair of 680's. And while I could do the 680's is the price per dollar really worth it? There isn't that much difference and will that tiny "upgrade" be worth the extra $$$?