Note:they use a very power hungry test setup-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.
your system is much more power efficient.your psu(xfx 650) provides 53 amps which is sufficient to run your system with 2x 7950.Mainboard
MSI Big Bang XPower X58
Core i7 965 Extreme @ 3750 MHz
HIS Radeon HD 7950 / AMD Reference Radeon HD 7970 / PowerColor Radeon HD 7950 PCS+
6144 MB (3x 2048 MB) DDR3 Corsair @ 1500 MHz
Power Supply Unit
Dell 3007WFP - up to 2560x1600