Here is Guru3D's power supply recommendation:
Radeon HD 7770 - On your average system the card requires you to have a 450 Watt power supply unit.
Radeon HD 7770 Crossfire - On your average system the cards require you to have a 550 Watt power supply unit as minimum.
If you are going to overclock the GPU or processor, then we do recommend you purchase something with some more stamina.