As others have stated, you don't need or want another 5850. Your cards only have to be the same when you're using crossfire. Crossfire treats 2 or more cards as a single card for increased performance, but you only get to use the displays from 1 card, and you don't need a 5850 to drive monitors.
This XFX hd6770
looks like it supports 5 display port monitors. If your monitors don't have display ports, you'll need active adapters
Have you checked how many extra PCI express slots your motherboard has?