Today I replaced the totally outdated Gentoo system on my desktop PC with Ubuntu "Edgy Eft".
When I tried to switch from the "vesa" to the "radeon" driver, the PC crashed on startup of the X server. After some internet search I found a corresponding bug report: This is actually a bug in the radeon driver. However there is a workaround: By setting the "AGPMode" option to "8" in the /etc/X11/xorg.conf the problem can be fixed. At least this perfectly worked for my ATI Radeon 9600 XT.