I've got a VX450 driving a 5850 using the built-in 6-pin and an adapter for molex-to-6-pin. Been running for two days now, no problem with DX9, DX10 or DX11. High of 66 degrees under full load, with fan at a quiet 29%. Life is good.
ASRock X58 Extreme
Core i7 920 (@3.6, stock voltage)
3x2GB OCZ PC3-10666
Cooler Master Hyper 212+
Cooler Master Storm Scout (120mm front, 140mm top)
WD Caviar Black 1TB
Samsung DVD ROM/R/RW