The next release of x.org should include support for the X800. Planned release date is Aug 25th 2004.
A patch from ATI was circulated that added support for the R420 (X800),
R423 (X800 PCIE), RV380/M24 (X600 PCIE), RV370/M22 (X300 PCIE) and
RS350 chips, Render support for all Radeon chips below R300 [already
integrated by EricAnholt] as well as some misc laptop, CRTC init and Xv fixes.