+1 what jsc said.
Here are the official power requirements for the ATI Radeon™ HD 5970 video card.
ATI Radeon™ HD5970 System Requirements:
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
650 Watt or greater power supply with one 75 watt, 6-pin and one 150 watt, 8-pin PCI Express® power connectors
850 Watt power supply with two 75 watt 6-pin, and two 150 watt, 8-pin, PCI Express connectors for ATI CrossFireX™ technology in dual mode
The power supply recommendations are for an entire pc system.