If you're thinking of using Blender3D, the proprietary nVidia driver is the only reliable game in town at the moment.
On the X.org wiki RadeonProgram support matrix, Blender3D support is recorded as GOLD for the older Radeon R300 series. Looking at the small print, this means "(Blender) 2.49 requires low impact fallbacks to draw all interface symbols (stipple lines for lamp types, etc), but that affects speed. 2.50 requiries changing triple buffer mode to something else, or unusable (app problem, it seems to happen with other brands and operating systems too)."
The more recent R500 R600 series chipset support is rated as GARBAGE/UNKNOWN. The current R700 is SILVER, which, being translated, means "(26 Oct 2009) [mesa-git] Crashes on many operations and does not update its interface correctly."
For Blender, Radeon is not quite there yet, in other words. Stick with programs marked PLATINUM in the support matrix if you actually need to get stuff done.