If you are sure you will only be using one XFX Radeon HD 5850 video card, then the Corsair VX 550 would be an appropriate choice. It is a high quality power supply with a single large +12 volt rail rated at 41 amps. It can easily power a system with any single video card made. Corsair power supplies have a reputaion for high quality. They consistently earn high marks in technical reviews. They are stable, relaible, and come with a 5 year warranty. The VX550 is currently available for $70.35 after mail in rebate. Unfortunately shipping is not free this week.
For a video card requiring two, 75 watt, 6 pin, PCI-Express power connections you would plug in the standard 6 pin connector and the 6 pin portion of the 6+2 connector which keeps it at the standard 75 watts.
For a power hungry video card that requires an 8 pin connection you would plug in all 8 pins of the the 6+2 connector. The 2 extra pins increase power from 75 watts to 150 watts.