The easiest way to go by is to look at the power requirements on the card. According to NV's
official specs, you need a 500W PSU minimum for a standard 760 (overclocked models would need more).
In practice, you can get by on with less power than that. You can look up GPU reviews on tech sites; they often have Total System Power data graphs showing the actual power used under peak load. But there are other technicalities to look into, such as the PSU design in terms (single rail vs dual rail) or quality (crappy ones won't deliver their advertised power, good ones can over-deliver, though I wouldn't recommend it).
tl;dr: Don't cheap out on the PSU. Get something with 500~600W from a reputable brand (I like Seasonic, personally).