Just check out the tested/supported power supplies for the 5850 in the ati website.. You'll come to know how many are single rail and how many multi rails.. Check out their specs to resolve your confusion..
For multiple rail PSU's you will have to find out the combined output for the 12volt rail. You will find this info on the sticker on the side of the unit or on the manufactures website under specs. It will usually give you the combined output in watts and to convert watts into amps just divide it by 12 so if it says combined output is 480 watts that would equal 40 amps. You will notice on power supplies with multiple rails will not output the total of all the rails it will be less so be careful when looking at power supplies there are a lot of cheap units that will not supply its rated power.
A few names of trusted PSU's Corsair, SeaSonic, Antec are just a few.
There has been some controversy over which is better multiple rail or single rail units, but if the unit is a quality unit it does not matter as long as it can supply the power safely and efficiently thats all that matters in the long run.
I am running rig 2 in my public profile with an Antec TPII 550 running an AMD 965BEC3 o/c to 3800 4 gigs DDR3, Radeon 5850 with BIOS mod to 765/1125 and it is perfectly solid. Ran Furmark stability test for over 2 hrs without a hitch.
A 1st class brand 550 PSU will do the job just fine (seasonic, corsair, cooler master, Antec, SIlverstone). From less reputable brands like CoolMax, Rosewill, Sigma etc, I would go with 650Watt min. I would honestly not go bellow that mark for quality.
Also look for 80+ certified as a minimum, not only are these PSU more efficient, they use higher quality parts.
Here are the official power requirements for the new 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.
Corsair and Seasonic are two brands that have a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty. Some of the newer models come with a 7 year warranty (in the USA).