Nvidias intended upgrade for the 5/6 series cards is the gtx 970. Considering you have a locked Intel cpu, and a 970 pulls less than 200w, has the same connectors as the gtx660, it'll be a direct replacement, even on a 450w psu. That's the best possible performance upgrade, without upgrading the psu as well.
On a side note I oc'd my 660ti and currently it's stable at slightly better than a stock Gigabyte G1 gtx960, which falls just below a gtx770 in performance. It's a free option if you feel like playing with it, and has increased my fps @ 5-10 depending on game.