I don't think i need this much power but I would like extra in case I ever did sli/crossfire. I have an overclocked i5 750 @ 4 ghz and a ssd, 1 7500 harddrive, 1 dvd/writer and a 7970 overclocked and about 7 fans(3 200, 2 140s, 2 120s). I will be upgrading this system once ivy bridge comes out.
For a system using two Radeon HD 7970 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 750 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 59 Amps or greater and have at least two 6-pin and two 8-pin PCI Express supplementary power connectors.
Seasonic 850 Watt power supplies have a +12 Volt continuous current rating of 70 Amps and also have at least four PCI Express supplementary power connectors so it should be sufficient.