You shouldnt get a bottleneck but why are you upgrading, instead of getting either of those cards, just wait for nvidia maxwell 8xx series or the next amd series. You would really waste your money if you upgrade to gtx 670 or 680 as it is only a minimal increase in performance but why get the gtx 680 when the better gtx 770(680 rebadge) performs better and also costs less. Unless your playing at XHD or UHD resolutions like 2560 x 1440 (1440p) or higher, i see no reason for you to upgrade but even then if you want to upgrade, i would say that get atleast a gtx 770 but i dont reccomend it. Your current gtx 660ti would be able to play all games fluently if you dont mind turning the eye-candy down a bit.