Supposedly it goes something like this in terms of performance;
HD6970 > HD6950 > HD5870 > HD6870 > HD5850 > HD6850 > HD5830
I agree that the shift in the numbering scheme is pointless and confusing, especially when leading with the mid-range cards.
FYI there is a huge thread on the subject already. These kinds of posts would be better made in that thread.