For a system running with a single Radeon HD 6790 graphics card a power supply with a combined +12 Volt continuous current rating of 24 Amps or greater and with at least two 6-pin PCI Express supplementary power connectors is recommended.
You may find power supplies on the market that supply more than enough Wattage to run the system. However, many of them lack the Amperage capacity on the critical +12 Volt rail, which is necessary to support that claimed Wattage.
The two devices that consume the majority of the power, within the computer, are the CPU and the GPU. Both of these devices obtain all of their power from the +12 Volt rail(s). If this critical rail is unable to supply enough power the system is not going to run properly. So don't just look at the power supply's total wattage rating.
The power supply calculators mentioned in the above replies don't provide this critical information. The eXtreme Power Supply Calculator Pro will provide the critical PSU rail specifications but only if you're willing to pay for it.
The Antec EarthWatts EA-500D Green with its combined +12 Volt continuous current rating of 37 Amps and with one 6-pin and one 6+2pin PCI-E Supplementary Power Connectors is more than sufficient to power your system.