The trick with AMD models is to break down the number (until there change the scheme again).

XYZ0

X represents the generation of the card. This tends to be the indicator of a smaller micron process for the GPU chip.
Y tends to represent the performance. The higher the better. Sometimes in tandem with X, this can mean a smaller chip from the previous generation, or a newer generation core architechure.
Z seems to represent the performance range within the Y family.

And from time to time, they change Y's meaning. I have a 5870. The 6870 is actually slower, and the proper upgrade path is a 6970.