"4850 Crossfire is about 40% faster than a single 4870, however bare in mind a few games aren't very optimized for multiple GPU's and so the 4870 may win in these rare incidents."
http://www.legitreviews.com/article/734/1/
The 4850 crossfire beats a single 4870 is most cases and performs similar to the 4870 crossfire in most games.