For a system using two Radeon HD 7950 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 700 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 46 Amps or greater and have at least four 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT a crucial factor in power supply selection!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important.
Even a Seasonic M12II-650 Bronze (SS-650AM), with its +12 Volt continuous current rating of 53 Amps and with its four (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your intended system configuration.
I think it worth as it is Modular, while the XFX is Semi-Modular.
Semi-modular here means that the cables that will be used in absolutely every build (the mobo, CPU and two PCIe power cables) are not modular, and everything else is. It's no different from a fully modular PSU when used in a build. My price difference refers to the cost after rebates.