Mesa r300g xorg
There is a xorg state tracker for gallium. This means that you don't need an xf86-video-* driver when you are using the xorg state tracker. This is an experimental feature so there could be problems when using this.
There is a package on aur which builds mesa with the needed state tracker for cards supported by r300g: mesa-r300g-xorg
You need to remove at least xf86-video-ati and all the mesa packages before installing the package.
Please add the following line to your xorg.conf in the "Device" Section:
Section "Device" ... Option "2DAccel" "TRUE" EndSection