Here is Guru3D's power supply recommendation:
AMD R9-290X - On your average system the card requires you to have a 550~600 Watt power supply unit.
AMD R9-290X Crossfire - On your average system the cards require you to have a 800 Watt power supply unit as minimum.
If you are going to overclock GPU or processor, then we do recommend you purchase something with some more stamina.
Here is Guru3D's power supply recommendation:
AMD R9-290X - On your average system the card requires you to have a 550~600 Watt power supply unit.
AMD R9-290X Crossfire - On your average system the cards require you to have a 800 Watt power supply unit as minimum.
If you are going to overclock GPU or processor, then we do recommend you purchase something with some more stamina.