5850 crossfire is generally considered to give better performance than a single 5870.
The more appropriate comparison should be to the 5970 where the costs are about the same, and the performance is similar.
I would favor a single card wherever possible, in this case the 5970. Your psu cost will be less, and you will have an upgrade path left.
I agree w/ the 5970 over the Xfired 5850's.....those solutions are $200 more than the 5870 option tho. The single card option also leaves room for a dedicated PhysX card should you decide that's a direction you'd like to take.