For a system using two AMD Reference Design Radeon R9 270X graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 700 Watt or greater system power supply. The power supply should also have a maximum combined +12 Volt continuous current rating of 43 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!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most critical factor.
Overclocking of the CPU and/or GPU(s) may require an additional increase to the maximum combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock being attempted.
The +12 Volt continuous current rating of 43 Amps or greater, that I specified above, will handle the running of FurMark stability testing and GPGPU usage with two overclocked AMD Reference Design Radeon R9 270X cards. It should also be sufficient to handle the overclocking of the i5-3570K to 4.4 GHz.