Flaggedbacon :
650 ti boost sli if you have the right PSU, it beats the 680, or if you can try to get. A. 7850 crossfire, in some cases it can be cheaper
i will say no to crossfire and sli for now, if you are playing at 1080p, and want smoother gameplay, go with gtx 760 on par with 670, and if you are playing above 1080p go with gtx770.