Here is Guru3Ds power supply recommendation:
GeForce GTX 760 - On your average system the card requires you to have a 550 Watt power supply unit.
GeForce GTX 760 SLI - On your average system the card requires you to have a 700 Watt power supply unit.
If you are going to overclock GPU or processor, then we do recommend you purchase something with a bit more stamina.