ATI cards work best, specifically the higher end 5000 and 6000 series cards like your 5850.
You can mix and match cards from different manufacturers, and of different makes and models.
For example, right this very second I'm using a 5850 and 5830 on this box mining in the background.
There is a fantastic entry on mining hardware comparsion on the wiki, here:https://en.bitcoin.it/wiki/Mining_hardware_comparison
But, be sure to spend some time browsing the more general category as well, which can be found here:https://en.bitcoin.it/wiki/Category:Mining