For a system using a single AMD reference design Radeon HD 7870 GHz Edition graphics card AMD specifies a minimum of a 500 Watt or greater system power supply. The power supply should also have a maximum combined +12 Volt continuous current rating of 28 Amps or greater and have at least two 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 Corsair VS Series VS550 (SKU# 75-001836 / CP-9020050), with its maximum combined +12 Volt continuous current rating of 42 Amps and with two (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with a single AMD reference design Radeon HD 7870 GHz Edition graphics card.