Here this chart shows the differences: http://www.anandtech.com/video/showdoc.aspx?i=3650
THe 5870 has more texture units and streaming processors. The GPU and Memory is also clocked higher. Chances are the memory and GPU are binned higher for the 5870 as well. Chances are you can OC the 5850 to the same speed as the 5870, but you are still working with less texture units and streaming processors at the same clock speed. The 5870 should also clock higher than the 5850.
You are paying $150 more for the higher end card even though it is not horribly faster, though it IS faster. It also has a bigger cooler.