A system with a single HD 7870 only needs a recommended 500w power supply. 600 watts is fine for 2 cards in Crossfire. As long as you buy a quality unit then this wil lcover overclocking as well.http://www.realhardtechx.com/index_archivos/Page362.htm
Quality brands include Corsair, Seasonic, PC Power and Cooling, XFX, Silverstone, Enermax, OCZ, Antec and the higher efficiency Rosewill units.
Post a budget and a place you would like to buy from and I wil try to find you a good deal.