I guess it depends on what cards you're running - i have a 5870, 5850 and their running happily on a 850 pulling around 430w with some other stuff running

(i have 2 ASIC USB miners plugged in)
Well I have a 5850 and a 6950 running right now... but there is an empty PCI-E slot that I want to get put another 5850 into, but I might shell out some cash and get a 7950 just becase I want one.
I think I will need to get a 1000W supply to run all three though... actually, I have a 630W laying around, so I could just run two PSUs.
if yiou have some spare BTC there are occasionally ones in the forum for sale - so far none in teh UK for me though