(In reply to erhard_f from comment #15)
> Played around a bit with xorg and kernel options on my G5 and worked out
> that I can drive the Radeon 9650 at AGP x8 if I disable DRI completely via
> Option "NoAccel" "true" in /etc/xorg.conf.
> 
> I had no GPU-lockups since then

Sure, because you're not actually using the problematic AGP functionality. :)

> and xorg is much more responsive despite lacking DRI hardware acceleration
> (radeon.agpmode=-1 with DRI enabled).

FWIW, the value of the radeon.agpmode parameter shouldn't matter if you disable
DRI (which BTW means no GPU hardware acceleration can be used at all).