For a system using a single Radeon HD 7770 GHz Edition graphics card AMD specifies a minimum of a 500 Watt or greater power supply. The power supply should also have a maximum combined +12 Volt continuous current rating of 23 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 @ 45°C - 50°C ambient temperature, is the most important factor.
Overclocking of the CPU and/or GPU(s) will require an additional increase to the maximum combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock you are trying to achieve.
Hi - I'll list a few that are in stock and will run a sys with a 7770 comfortably:
You don't need to spend $150 US, but most of the better priced units
for your sys are out of stock. You could try other sites.