May I ask, then, why you are going for such a high wattage?
The important specs on a PSU are, in this order, the quality of the PSU, the +12V amperage, then lastly the total wattage.
A shady, cheap PSU can cause system instability and random crashes, or worse, ruin internal components when it fails.
Even 550w from a quality PSU would be more than sufficient for a single HD7870.
Seasonic, XFX, Corsair and Antec are known for making quality PSUs.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139027
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139028
http://www.newegg.com/Product/Product.aspx?Item=N82E16817207013