One ati 5870 or two 5850 in crossfire?
Any help would be greatly appriciated
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.