All that's needed for this is a quality-made 550W power supply:
With one 7950 under full load in their system, their PSU pulled 300W from the wall outlet. The 7950 by itself pulled 138W, so that would make the PSU pulling 438W from the wall outlet in Crossfire. Their CPU was idling, so I have to add 150W for an overclocked 3570K which makes the PSU pulling 588W from the wall outlet. So if the PSU is 85% efficient while pulling 588W from the wall outlet, then the system is pulling 500W from the PSU. Your actual gaming power draw will be closer to about 400-450W.
Although, you'll need 4 x 6-pin PCI-E cables, so I recommend a quality-made 650W power supply. I don't go by brand names though. Can you order from Newegg? How much can you spend?