Radeon HD 7950 - On your average system the card requires you to have a 500 Watt power supply unit.
Radeon HD 7950 Crossfire - On your average system the cards require you to have a 700 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.
System in IDLE = 165W
System Wattage with GPU in FULL Stress = 300W
Difference (GPU load) = 138W
Add average IDLE wattage ~3W
Subjective obtained GPU power consumption = ~ 138 Watts