That system could run easily on a 550 W power supply, a 1000 W power supply is simply a waste of money for that system. Even if you are planning on running two 7950s I wouldn't go any higher than 850 W. Very very few units actually need a 1kW or larger power supply.
The 7950 doesn't use much power. You could get by easily on an XFX 550w.
If you are going to use a cheap generic knockoff PSU then you might want to stick with 1000w, but if you get something with real quality then even 550 is overkilling it unless you intend to OC as much as possible.
For a system using a single Radeon HD 7950 graphics card AMD specifies a minimum of a 500 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 30 Amps or greater and have at least two 6-pin PCI Express supplementary power connectors.
For a system using two Radeon HD 7950 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 700 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 45 Amps or greater and have at least four 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Combined Continuous Power/Current Available on the +12V Rail(s) @ 50°C ambient temperature, is the most important factor.