There are two questions for you to answer, basically:
If you buy a new GPU, when are you getting a new mobo+CPU?
This year? Get a 760.
Next year or later? Would you be willing to get a new GPU at that time to maximize performance (250$), and a "cheaper" (150$) one now?
If yes, get a 660 or 750Ti now. If not, get a 760, it'll still be good next year and the following.