For a system running a single Radeon HD 6950 graphics card the power supply should have a combined +12 Volt continuous current rating of 34 Amps or greater and with at least two 6-pin PCI Express supplementary power connectors.
If you're going to be overclocking the CPU and GPU then you should be considering a power supply unit with a combined +12 Volt continuous current rating of at least 38 Amps or greater.
It's not the power supply's wattage that determines if it is able to properly power the system. The power supply unit's combined +12 Volt continuous current rating specification is the most important. The +12V rail(s) supply all of the power used by the CPU and GPUs.