I think a nice 550watt corsair would handle a single 5870 just fine, even with overclocking on the cpu, gpu, ram and all your peripherals. I wouldn't count on getting a second card for xfire though.

PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin Connectors for ATI CrossFireX™ technology in dual mode)
Certified power supplies are recommended
Minimum 1GB of system memory
Installation software requires CD-ROM drive
DVD playback requires DVD drive
Blu-ray™ playback requires Blu-ray drive
For an ATI CrossFireX™ system, a second ATI Radeon™ HD 5870 graphics card, an ATI CrossFireX Ready motherboard and one ATI CrossFireX Bridge Interconnect cable per graphics card (not included) are required
http://vr-zone.com/articles/suggeste....html?doc=7470