Our test system contains a Core 2 Duo E8400 Processor @ 3.0 GHz (FSB 1333), the ASUS P5E3 Deluxe mainboard, stock cooling on the CPU, DVD-rom and a WD Raptor drive. The results:
Videocard
System Under FULL load
Radeon HD 4850 269 Watt
Radeon HD 4870 334 Watt
Radeon HD 4850 Crossfire (2) 421 Watt
Radeon HD 4870 X2 (R700) ** 448 Watt
Radeon HD 4870 Crossfire (2) 480 Watt
** this is an early beta engineering test-sample, the power saving features are disabled on these boards meaning that IDLE wattage and even peak wattage sill be off by a good number.
The monitoring device is reporting a maximum system wattage peak at 450 Watts with 2-way 4870 embedded on the X2. Our IDLE wattage was 260 Watt, way too high but again .. power saving features are disabled on these boards, so that will drop significantly on the final product.
Power supply wise I recommend at the very least, 750-800 Watts. You might not utilize all that power distribution, yet especially in this high-end game make sure you have some reserves folks. You do not want your PSU to run at it's maximum all of the time. See, it's not that your PC will consume that much power, it's just that you want to make sure your PSU can deal with the hefty load and will stay stable during you entire gaming experience.
Please make note of the fact that the card uses one 6-pin PCIe power connector and one 8-pin connector.