There are some things you have to consider.2 GTX 470s will perform far better than 2 HD 5850s.But they will also consume much more power.Still the GTX 470s are a better option. Make sure that you have a good 850+ W PSU for the job.
You have to decide whether you want 3D or PhysX, but you can do PhysX on ATI+ Nvidia combination with driver mods.But I do not think that it is a good idea.
yeah i have heard about putting a nvidia with ati but i am very skeptical about it. any more suggestions for gpu, is there a 3d vision for ati