I know I am cutting it close here and this setup is not recommended, but COULD a xfx 550w psu be enough to power two 7870 ghz edition graphics cards in crossfire along with my i5 3570k(not overclocked)???
For a system using two Radeon HD 7870 GHz Edition 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 Combined Continuous Power/Current Available on the +12V Rail(s) rated @ 45°C-50°C ambient temperature, is the most important factor.
Overclocking of the CPU and/or GPU(s) will require an additional increase to the 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 you are trying to achieve.
The XFX 550W Core Edition Full Wired (Bronze) (P1-550S-XXB9), with its combined +12 Volt continuous current rating of 44 Amps and with one 6-pin and one (6+2)-pin PCI Express supplementary power connectors, is electrically sufficient to power your system configuration with two Radeon HD 7870 GHz Edition in 2-way CrossFireX mode. It is deficient by two PCI Express supplementary power connectors so you will have to resort to using the dual 4-pin Molex peripheral to PCI Express power adapter cables that may be included with the graphics card.