Both are from the manufacturers "Builder" series which IMO is not well suited for a an "enthusiasts" (read overclockers) box.
Looking at what AMD sayshttp://www.amd.com/us/products/desktop/graphics/amd-radeon-hd-6000/hd-6850/Pages/amd-radeon-hd-6850-overview.aspx#3
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard500 Watt or greater power supply recommended
Certified power supplies are recommended. Refer to http://ati.amd.com/certifiedPSU for a list of Certified products
If I didn't meet manufacturer's recommended system requirements, I'd be concerned about sending in my sales receipt for a warranty claim if the PSU on the same receipt didn't meet those requirements.
Personally, I look for a minimum 9.5 jonnyguru rating and 1% voltage variation under load for stable overclocks ....it's not just about providing enough power, it;s about being able to provide rock stable voltages while outputting that power under widely varying loads in order to get the best OC's. The smallest PSU I will use is a 650 watter as you really don't save much money going smaller w/o sacrificing quality (good sales excepted)
Of course, f ya not OC'ing anything, then an 8.5 rating will more than suffice.