For a system using a single Radeon HD 6770 graphics card AMD specifies a minimum of a 400 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 25 Amps or greater and have at least one 6-pin PCI Express supplementary power connector.
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 @ 50°C ambient temperature, is the most important factor.
The Corsair Enthusiast Series Modular 550W (CMPSU-550TXM), with its combined +12 Volt continuous current rating of 45 Amps and with two (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with a single Radeon HD 6770.