For all intents and purposes, consider the Radeon 9100 as a Radeon 8500LE. It uses the same core (the R200 core) and their clock speeds are very close to each other.
The RV280 is an AGPx8 upgrade to the RV250, the core used in the Radeon 9000 and 9000 Pro. It's still unclear whether the RV280 will be made to a .15 micron process or a .13 micron process; if it's the latter, expect a pretty nice performance jump over the Radeon 9000/9000 Pro, and a cost of around the Radeon 9000 and 9000 Pro's prices (around $100 for 128mb version).
If you can wait a month before buying a graphics card, then do so. In March both ATi and nVidia will release a total of 5 different cores (6 if you count the NV30), which will lower prices of existing cards and provide more buying options.