Well the exact needed kinda depends on the gpu, and that depends on your needs and budget. For the price of a 960 you could get a gtx 1050ti and be quite comfortable with a good 450w unit, whereas for a little more, the Rx470 would also be a good option at 450w. If going for slightly older cards, say from ebay, you'll need to start thinking about a good 550w. Most of these are subjective, recently there's been many 650w models actually cheaper than the exact same model in 550w.
Seasonic S12-II or M12-II 520w I, Antec HCG-M 520w, Evga GS or G2 550w, XFX TS/Pro 550w (not XT) Corsair CXM 450W/550W are all very good, which ever is cheapest on sale would be best value.