Hey,
So, quite obviously, the GTX 670 is the best choice, when you have a single GPU. But, it is very expensive and isn't much better then the GTX 660. The GTX 660ti is not really worth the money, it usually costs about 100$ more and isn't so much better. One thing you would really recommend if you have enough space, is keep the GTX 660 and buy another one and SLI them together. That would be much more efficient and it would be comparible to the gtx 680 (with even some things better then the gtx 680). Also, I think you should buy the Asus model because it is heard to have a lot better cooling and has pretty good gpu tweaking software...but that is just my opinion.
-Alek
