Hallo!
I have an hp laptop with an ati chip, and I had to create the tmpfs entry in /etc/fstab to make it work:
tmpfs /dev/shm tmpfs default 0 0
thanks for the hint - but I already have this entry ... :(
I believe you're still using the linux kernel radeon driver (the vendor string from glxinfo should be ATI).
AFAIK, with ati cards you should also be sure that you're loading and using the correct agp module (the one you've got installed with your ati driver).
hm ... I have to check this ...
LG Georg