[I hope it is not too off topic] I found a nice HOWTO here http://en.tldp.org/HOWTO/XFree86-Video-Timings-HOWTO/index.html that explains in deep how to set ModeLine entries for XF86Config. It talks about two automatic ModeLine generation tools:
KVideoGen - http://paranoia.rulez.org/videogen/ [a CGI script] - http://zaph.com/Modeline None of these links seems to work, could it be for copyright problems? Thnx PAolo
