My understanding is that r300g is the new default in Mesa 7.9. See http://www.mail-archive.com/mesa-commit@lists.freedesktop.org/msg23390.html . Does this mean that Arch will automatically switch to it on release of Mesa or will extra steps be required?
As I understand it the PKGBUILD will also have to be altered. The driver will be built by default but not installed to "/usr/lib/xorg/modules/dri/" as mentioned by Stafano. The purpose of having both drivers built is so that one could experiment with the gallium driver with export LIBGL_DRIVERS_PATH = ... before executing particular opengl programs without switching outright. I've been doing this for a couple weeks now with the git version of just the r300g driver running with Arch's packaged mesa 7.8 and have found that it has fixed all the bugs I was having in Neverwinter Nights and eduke32 with no new problems besides the occasional stuttering when fps drops too low. I would vote for making it the default in Arch with mesa 7.9.