For a system using two Radeon HD 7770 GHz Edition graphics cards in 2-way CrossFireX mode AMD specifies a minimum of a 600 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 30 Amps or greater and have at least two 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Combined Continuous Power/Current Available on the +12V Rail(s) rated @ 45-50°C ambient temperature, is the most important factor.
The Seasonic X-Series 650W, with its combined +12 Volt continuous current rating of 54 Amps and with four (6+2)-pin PCI Express supplementary power connectors, is way more than sufficient to power your system configuration with two Radeon HD 7770 GHz Edition in 2-way CrossFire mode.