I believe the auto-config programs found in LinuxPPC are not packaged for Debian. They make some RedHat assumptions about file locations, so they won't run out of the box on a Debian system, and no one I know of has ported them.
So, it's you, your text editor, and /etc/X11/XF86Config . If you're using a framebugger, "fbset -x" (I think?) will print out your current mode settings in a format you can just paste into the XF86Config.
