For a
system using a
single reference clocked Radeon HD 7870 (Boost Edition Tahiti LE) 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 35 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) will 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
Seasonic S12II-620 Bronze 620W power supply (SS-620GB), with its
maximum combined +12 Volt continuous current rating of 48 Amps and with
one 6-pin and one (6+2)-pin PCI Express supplementary power connectors, is
way more than sufficient to power your system configuration
with a single Radeon HD 7870 (Boost Edition Tahiti LE) graphics card.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151096
The
XFX 550W Core Edition Full Wired (Bronze) power supply (P1-550S-XXB9), with its
maximum 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
more than sufficient to power your system configuration
with a single Radeon HD 7870 (Boost Edition Tahiti LE) graphics card.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817207013
Both power supplies have more than enough spare capacity to handle overclocking of the CPU and/or GPU.