If it's a reliable 800W PSU your fine no worries.Here is Guru3D's power supply recommendation:
Radeon HD 7950 - On your average system the card requires you to have a 500~550 Watt power supply unit.
Radeon HD 7950 Crossfire x2 - On your average system the cards require you to have a 700 Watt power supply unit as minimum.
Radeon HD 7950 Crossfire x3 - On your average system the cards require you to have a 850~900 Watt power supply unit as minimum.
If it's a reliable 800W PSU your fine no worries.Here is Guru3D's power supply recommendation:
Radeon HD 7950 - On your average system the card requires you to have a 500~550 Watt power supply unit.
Radeon HD 7950 Crossfire x2 - On your average system the cards require you to have a 700 Watt power supply unit as minimum.
Radeon HD 7950 Crossfire x3 - On your average system the cards require you to have a 850~900 Watt power supply unit as minimum.