+1 I pretty much agree with the above post.
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 i do recommend you purchase something with some more stamina.Just remember as much as the power (Watts) requirement matters, the current requirement, measured in Amps is as important if not even more important. This is especially true if you have an or several dedicated video card(s). As a general rule of thumb, you are aiming for the highest number on the 12V line.