I reckon 700W is pretty overkill, a system like yours wouldn't consume that much power, no where near. An o/c i7 system with two HD 5850s in CrossFireX used less than 500W on full load, at legitreviews, and you're only going to use one. Though, to be safe and keep options open, I'd get a Corsair HX650W, which has four 6+2 pin PCI-e power connectors, meaning that if you want another HD 5850 later for CrossFireX, it's possible. It has a strong single 12V rail with 52A on it, single is better than dual in my opinion, because single rails don't have to deal with load balancing. Higher quality PSUs tend to have a single 12V rail, like Corsairs and SeaSonics, whereas most other cheaper PSU companies like OCZ instead just use dual rails, for example on their ModXStream series.
Here are the official power requirements for the new ATI Radeon HD 5850 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.
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. Lately we've been seeing a few other brands offering some high quality units. One example would be the Antec Earthwatts and the Antec TruePower New models which are a major improvement over Antec’s older psu’s like the Basiq models.
Here is a link to an excellent explanation of multiple +12 volt rails: