You don't need a ridiculously huge PSU for a 5850. If you have a relatively normal PC ( i5/i7 8 gigs, air cooler, single HD, DVD/blu-ray, 5 or 6 USB devices, 2-3 1500 rpm 120mm case fans) then a 550 will do just fine *IF* it's not cheap and can actually supply 550. Such a PC will pull around 350 watts (including the 5850) and as long as the PSU is actually capable of supplying 550 watts during normal operation you won't have any problems.
But a lot of PSUs will say "600 watts" or whatever and what they really mean is that 600 watts is the most you can pull out of it before it overheats and (hopefully) shuts down. The real max "safe" load is actually lower than that on these cheap PSUs.
It does not pay to skimp on your PSU, you don't necessarily have to get a huge PSU, but you shouldn't get a cheap one either.
I use Antec PSU's (Antec is on Intel's recommended PSU lists for their motherboards), someone else can probably suggest other good brands to look at.
I've been using this PSU on my i7 860 with a 5850 with no problems at all. http://www.newegg.com/Product/Product.aspx?Item=N82E16817371016 Edited, Aug 27th 2010 4:05am by Lobivopis