Your processor uses at most 70w and if you bought the 750ti it uses 66 watts. That's not even 150. In comparison my top end card only uses 190w. Like I said, a 520w quality PSU can and will handle any single GPU solution.
For instance, one user here used to run a 3770k and GTX 690 on a Seasonic 550w G series Gold. That's an 80w processor stock, and stock the card can consume almost a little over 400w. Higher quality PSUs due to their efficiency rating and higher quality parts also have quite a bit of overhead. The 520w Seasonic M1211 probably has about a 200w overhead when peaking. Higher quality units are also rated to what they can continuously provide rather than peak power that will make them go boom like budget and **** units.
The GTX 760 is actually a good deal better than the 750ti. It is a big difference in power.