You could probably get away with a GOOD 650w PSU. But I'd probably go with a 700-750W since not many 650's come with (4) 6 pin PCI-E cables. It also depends on the rest of your system. If you have a highly OCed CPU, I'd stick with the 700-750. IF you are all stock a 650 would be plenty. Just remember you have to take in account the rest of your rig. This is what I would use, some people may say you can use a 550-600, but I always try and stay above the system requirements. Anything over a 750 will just be pointless.
I have the corsair TX650 and a 5850...will I be able to use molex to pci-e adapters to crossfire later? Everyone says not to use them(molex to pci-e)...why?