Thanks for all the replies!
When I do upgrade the motherboard, I will definitely get 2 PCIe slots and that way I think I could potentially crossfire a 2nd 5850 when a single one is no longer sufficient, although I have zero experience in that sort of thing.
At the local place I'm looking for all these parts, for PSUs, they have Enermax, Antec, and Coolmaster. I was looking at the ANTEC BASIQ BP550 PLUS 550W ATX12V 2.2 because it's only $80