just make sure the psu has 550W (or more) on the 12V rails and you'll be fine. (250 for the cards, 150 for the cpu for really high OC and then a little extra)
+1 for what theAnimal said. Each 4870 video card will require two PCIe 6 pin power connections. Corsair and PC Power & Cooling are both excellent choices.
You could in theory use adapter cables to convert 4 pin molex power cables into 6 pin PCIe power cables but I don't recommend it. You would be better off purchasing a power supply that meets your requirements.