well i do know that eyefinity cannot spread across multiple gpu's. they all need to be hooked up to single GPU.
It depends, when it comes to the 5850's you can only do 3 screens AFAIK (not sure if 6 screens can be achieved while having the cards active/non CF mode). When it comes to the Eyefinity 6 GPU's you can do up to 24 screens with 4 GPU's.
So if the OP wants 6 screens he will need an Eyefinity 6 GPU unless someone can prove otherwise..