Almost every ATI board is supported by xorg-x11, at least for the 2D accelerated features. The 3D support is provided either by the DRI project, which is already present inside the xorg-x11, or by closed source drivers provided by ATI.

The multimedia features are actually supported by the GATOS project, the gatos drivers will be merged in the xorg tree shortly.

You have almost the same x11 support on ppc or alpha platform but you can't use the ATI closed source drivers. That means that you can't use the r300 3d features at all. If you have such a board and you want it supported by x11, you should contact ATI and ask them to open the specs.

It should be but you may have a configuration issue due to the OEM PCI id that such chips may have. In most cases you may have to write the configuration file yourself or use the xorgconfig.

The xorg-x11 ebuild provides the X11 implementation. The kernel modules required by DRI are provided by the 2.6.x kernels, if you are still using the 2.4.x you should emerge the x11-drm ebuild, and ati-drivers provides the ATI closed source X drivers and kernel modules, for 2.4 kernels and 2.6 ones.

If you want just the Rage128 drivers and modules installed with x11
# VIDEO_CARDS="rage128" emerge x11-drm
To have just the Radeon support
(r100, r200, r250, but not r300 yet)
# VIDEO_CARDS="radeon" emerge x11-drm
To install the ATI closed source drivers
(r200,r250 and r300 only)
# emerge ati-drivers
To install just X11 but not install
any kernel module
# emerge xorg-x11

It is suggested the use of xorgcfg, xorgconfig or directly by using the Xorg auto configuration option:

# X -configure

For more information on how to get a basic xorg.conf configuration file, please refer to the Gentoo Desktop Documentation Resources.