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.
I use the Furmark power consumption value of the overclocked card, and the +12V current used in a standard reference system by graphics card makers, in deriving the +12V current requirement because I know that there will be those that will run a GPU stress testing utility on an OC'ed card. If the power supply is unable to handle the GPU stress test it's going to shut down or maybe even fail catastrophically.
ko888 - Thank You! Much appreciated. Thank you for the explanation too. As luck would have it I happen to speak, read, and write German fluently.
There are so few review sites out there that actually have the technical competence and ability to know how to properly measure graphics card power consumption irrespective of the rest of the components in the system.