For a system using two Radeon HD 7870 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 600 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 39 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 Continuous Amperage Available on the +12V Rail(s) is the most important factor.
If you're overclocking the CPU you will need to add some extra amperage to the combined +12 Volt continuous current rating recommended above to handle it.