2 Different 4850s work the same in crossfire?

They would perfect if they have identical clocks.
Otherwise the faster one would just slow itself down to the level of its slower partner, yet the crossfire would work indeed.

If both have the same GPU/RAM clocks, you are supposed to be able to CrossFire them just fine. If not the same clocks, I think they just use the lower of the 2 speeds, but it will still work.

(hsetir bet me to it)

Same advice as given above. 2 cards different makes but same clocks = success