why would you risk getting something that can perform as low as a 4850 (comparison to the 285 I'm saying).
If both cards are par, or the single GPU is slightly under, I would never go for the 2 weaker cards. It just doesn't flow for me
Remember the 285s can be found for 300$ if your lucky enough, and like you said they run quieter, can OC like a beast, runs very cool, and has the ability to run sli and tri sli, which is alot safer than the 4850s quad Fire.
Both these cards have seen to exchange blows, and for some reason the 4850 doesn't perform as well as 2 4850s from what I've seen.