The 58xx series cards have a lot more stream processors than the 68xx series, so they produce higher MH/s rates. I have been testing a 5850 and at 850MHz on core i get 330MH/s. I have just brought a 6870 which i will be testing tomorrow, i can report back tomorrow at to the difference.

5850 = 1440 stream processors
6850 = 960      "              "
6870 = 1120     "             "