When running the more-demanding Nexuiz game at 1024 x 768, the open-source driver "out of the box" on the X800XL was at 77% the speed of Catalyst. When disabling the swap buffer waiting, the Gallium3D driver was faster by 17%. There is a caveat, however, and that is with the current Mesa code there were some black textures when running Nexuiz on the X800XL. The X1800XT rendering was correct, but for the older GPU there were some differences, which may explain why the open-source driver performed more competitively for the R400 ASIC.
The open-source Radeon X1800XT graphics card was running at 52% the speed of Catalyst or 78% with the performance tweak.
When upping the resolution to 1920 x 1080 for Nexuiz, the X800XL graphics card was at 69% the speed of Catalyst or 77% for the Radeon X1800XT graphics card.