To find information regarding support for a particular graphics card/chipset use the following link,
http://xorg.freedesktop.org/wiki/Projects/Drivers?action=show
If you are using intel driver use this direct link to the manual page,
http://xorg.freedesktop.org/archive/X11R7.0/doc/html/i810.4.html
A modeline generation tool at the following link might be helpful while configuring your monitor.
http://www.bohne-lang.de/spec/linux/modeline/
