X11 config file must be updated (the name of this file depends on your X11 version, e.g. for xorg
/etc/X11/xorg.conf, I don't remember filename for XFree, however you can locate it in /etc/ directory)
Syntax of this config file is simple enough.
Also look at:
http://slackwiki.org/LCD_Monitor_Configuration_In_X.Org