Guru3D posted a Radeon 4850 and 4870 Crossfire review. Since Radeon series 4800 cards offer so much value, it might even be interesting to pair them in CrossfireX mode.

Much like NVIDIA's SLI offerings, AMD's ATI solution can be paired and matched as well. So pretty much today we'll place several cards together in CrossfireX mode, first the cards paired (two) and then we'll for the sake of it see if we can pair three cards CrossfireX mixed, meaning 2x 4870 and one 4850. When you compare the results to NVIDIA's latest high-end single card offerings, the results are a little shocking.

You can read the article at this URL:
http://www.guru3d.com/article/radeon-hd-4850-and--4870-crossfirex-performance/