The X300 ("ten-three-hundred"), X500, and X700 seem like a reasonable progression from 9500 and 9700, and allow them to speed-bump to even numbers with the refresh. I prefer the simpler "Radeon X7," though--or X8, if they'd rather the refresh go to X5, X7, and X9. Sounds rather car-like, which is not a bad connection for consumers to make, as the cards I'd associate with alphanumeric designations tend to be luxury (beyond the obvious Bimmer trucks, you've gotchyer Mercedeseses and Lexi and Audii, etc.).
Either way, they don't have much room for the next gen, no? Unless they go to X12, X16, and X18, which might be slightly confusing to customers. Or not. That way their next gen can just be XXyy ("The extra X stands for eXtra eXtreme!").
They'll have to come up with something new for the generation after that, though, or risk a hefty FCC fine and a healthy share of public indignance.
OTOH, they could just spin it as a sign that gaming is no longer a "kiddie" activity. So many possibilities....