You gotta remember the 4850x2 is literally like 2 4850s crammed onto one board, so of course it's going to out perform most single cards, even the 4890.
Now.. the 4890 and a 4870 in crossfire would probably smoke the 4870x2. but that'll be like $250 for the 4890 and like $190 for the 4870 so like $440 bucks right now, before rebates.
And honestly if I were to spend $440 on a crossfire system like that, I might as well just go with a 4870x2 which will be plummeting in cost soon I think, and whenever (ifever) the 4890x2's come out everything's gonna drop again.