the 5570 has pretty good performance for the power consumption, and is probably your safest option, but it's overpriced at the moment and you can find better performing cards at the same price if your willing to risk their higher power consumption.
you should take a look at this article, which compares what i think are some of your best options.
ATI Radeon™ HD 5570 System Requirements
Suggested ATI Radeon™ HD 5570 512MB DDR3 System Requirements
* PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
* 400 Watt or greater power supply recommended
* Certified power supplies are recommended. Refer to http://ati.amd.com/certifiedPSU for a list of Certified products
* Minimum 1GB of system memory
* Installation software requires CD-ROM drive
* DVD playback requires DVD drive
* Blu-ray™ playback requires Blu-ray drive
AMD's gonna be a bit concervartive given the number of crappy PSU's out there. I'd put my system through this calculator and let it "do the math" for you: