500W (or greater) power supply with one 75W 6-pin PCI Express power connector recommended7
600W power supply (or greater) with two 75W 6-pin PCI Express power connectors recommended for AMD CrossFire™ technology.
NOTE: Minimum recommended system power supply wattage is based on the specific graphics card and the typical power requirements of other system components. Your system may require more or less power. OEM and other pre-assembled PCs may have different power requirements
Radeon HD 7850 - On your average system the card requires you to have a 500 Watt power supply unit and 650 Watt if you go with two cards in Crossfire mode.
Radeon HD 7870 - On your average system the cards require you to have a 550 Watt power supply unit as minimum and 700 Watt if you go with two cards in Crossfire mode.
If you are going to overclock the GPU or processor, then we do recommend you purchase something with some more stamina.