It depends what you want. Usually a single better card is better and easier to deal with. Then it is the value. Today you can buy a HD5870 for ~$400 but 2 x HD5770 in crossfire deliver about equal HP for a total cost of ~$300-320
Artifacting from crossfire isn't really a problem these days. Usually a driver problem but the odd game would have it.
Just remember a single card runs cooler and quieter than two cards.
Some games scale really really good with crossfire and SLI but some games don't. While all games scale equally as well on flat out better cards.
I see... Thanks for quick reply...
If the price of the Eyefinity edition will not be very different from normal one's, then I could buy it too...
I set a budget for and 5870 + 3 monitors already and I currently think 6 is too much... but who knows?