For me with Slackware 10 all I had to do was download a snapshot of the radeon driver from http://freedesktop.org/~dri/snapshots
. For some reason the've stopped posting the latest ones, but the latest one from the archive will do.
Also, before you try this, check if you can sucessfully load the module "agpgart". For some reason the kernel has issues recognizing the AGP, so if it doesn't successfully load you'll have to find a kernel patch. More information is available here
I'm not quite sure why that worked with slackware... Either they merged the AGP patch into the 2.4 tree at some point or Slackware added the patch themselves... Oh well, either way it works for me.
NOTE: These instructions work for the 320M and 340M, but I'm not sure about the 345M. Also, even when you do get it working, ATI hasn't released the specs of the hardware, so the open source drivers are pretty crappy and you can only get about 400-500 fps in glxgears. Its enough to play tuxracer and stuff like that, but not enough for games like UT2003/4.