Well the 5850 is a lot faster than the 6850.What makes the 6850 better is only it's crossfire performance.Scaling has been improved a lot with the new 6000 series GPU's.So x2 6850's will actually beat a pair of 5850s.Also the improved tesselation with the 6850.But after all of that the 5850 is still faster as a single card.Also Diablo 3 won't be DX11 so you won't need to worry aobut the 6850 performing better.
5850's the best choice. Also, if you haven't bought your mobo yet, get one with two PCI-e slots at 16x/4x and you should be able to run crossfire and add a second 5850 for awesome performance down the line. I think the BP550 might handle that, but you'll want to double check that.
Yah the only concern I had is how old your power supply was.Since it's brand new it should last a long time and you shouldn't have any problems with it.I wouldn't advise you to run two 5850's off of it tho,you will be pushing it's maximum capabilites thus reducing it's life span.
I have an 850W PSU and based off of my Kill-A-Watt, I never exceed 570W. Of course, I plan to replace my PhysX 8800GT with a second GTX 470, then I'll get closer to 700W and can put the PhysX card back in to get closer to 850W. And I overclock it all, which you won't want to do if you crossfire.
If you do decide to crossfire get a board thats x8/x8 not x16/x4.And yes i'm sure you can crossfire 5850's with that PSU but it's not reccomend because you will be running it near it's maximum and that will degraade it's life span.If this GPU upgrade is based on a crossfire setup then going with the 6850 would be a better choice.