in quad-fire you would be able to use only one monitor.
without quad-fire and with 2 4850s, I believe you would be able to run 3 monitors
A more viable soulution for you would to be a 5xxx series card (3 monitors for one card)
I'm already running one 4850X2, and I have a chance to P/U another one for about a $100. Yes, I wish I would have got a card that supported eyefinity instead of this card, but it's what I have to build on...
If you're not planning to game, just use extended desktop, I don't see why you can't max out the outputs and have 6 monitors going. But then I don't have experience in that. (In this scenario you wouldn't quadfire, just have 2 standalone cards)