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.
Perfection - Seasonic X Series
Close to Perfect - Corsair HX, EVGA SuperNova G2, Seasonic M12
Can Do the Job Well - XFX Core Edition, Antec HCG / HCL, Seasonic S12