Take the 3870 for example, or to use the full marketspeak, ATI Radeon HD 3870. The first number, 3, denotes the Generation. Why this is different from the 2x00 line I don't know, but from now on, this should advance generationally, whatever that means.
The second number, in this case x8xx, connotes Family, and the last two digits, xx70 is the Variant. 70 is equivalent to XT, 50 is the new name for Pro, and 13 is the card from that company you never heard of with the Englishish manual and no web site. Don't buy the 3813.
When the next gen variants of the 2400 and 2600 come out, expect to see a 3x50 and 3x70, and likely with a few other variants as well.
Last up, there is a card named ATI Radeon HD 3870 X2 on the slides that we have seen, with a release date of 'winter'. The non-X2 line has a price listed as $150-250, while the X2 is in the same price point as the 2900XT, or about $400. I guess that would make the X2 two higher end 3870s on a PCB, but that is not specified.