If you are getting the GTX 900 series, they pull between 130 and 150 watts and require a minimum PSU of 500W.
GTX 770-780ti ~ 250 W requiring a min 600W PSU
GTX 760 - 170W req. min 500W PSU
GTX 750 - 55W req. min 300W PSU
Everything extra you put in a PC adds to the total power usage, some more than others. Search Power Supply Calculator. Also make sure your power supply has enough (if needed) extra cpu 4/8 pin connectors.