Depends what you want to play? Crysis, WOW, doom 2? Its all relative to what you intend to use those graphics cards for. 2 4850s are on par with one 4890, but you can get a 4890 for less with rebates now.
Funny thing is the OP never came back to give us the needed info so we could make an informed recommendation. And the original question is kind of unclear. Everyone knows 4890 in crossfire will outperform 4870 in crossfire, but if you throw value into the mix, the 4870s are a far better value, since you can get two for slightly more than the cost of one 4890. So now the thread has been taken over by the obvious posts of 4890 in crossfire is the best performance.