Dear Christian,
I have just installed and booted kernel-image-2.4.25-1-686 package and the XFree96 log now sais that it happily allocated IRQ16 for DRM operation.maybe a newer kernel would help, there was some drm/dri merge somewherein 2.4.23 or 2.4.24 ?
Thanks for your kind and patient :-) help. Gabor