For a system using two Radeon HD 7770 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 maximum 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!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most critical factor.
Even a Seasonic S12II Bronze 430 Watt power supply is sufficient.
Seasonic S12II-430 Bronze
• maximum combined +12 Volt continuous current rating of 30 Amps
• one 6-pin and one (6+2)-pin PCI Express supplementary power connectors