First, the system requirements!
Here are the official power requirements for the ATI Radeon HD 5850 and HD 5870 video cards.
ATI Radeon™ HD5850 System Requirements:
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
500 Watt or greater power supply with two 75 watt, 6-pin, PCI Express® power connectors.
600 Watt or greater power supply with four 75 watt, 6-pin, PCI Express® power connectors for ATI CrossFireX™ technology in dual mode.
ATI Radeon™ HD5870 System Requirements:
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard.
500 Watt or greater power supply with two, 75 watt, 6-pin, PCI Express® power connectors.
600 Watt or greater power supply with four, 75 watt, 6-pin, PCI Express® connectors for ATI CrossFireX™ technology in dual mode.
The power supply recommendations are for an entire pc system.
Second, the power supply!
The XFX power supply you linked to is a very good psu. It should not have any problems powering your system with two 5850's. Here is a link to a competent technical review:
http://www.hardwaresecrets.com/article/899