aoane@aoa-nux:~$ sudo dpkg-reconfigure xserver-xorg
[sudo] password for aoane:
xserver-xorg postinst warning: overwriting possibly-customised configuration
   file; backup in /etc/X11/xorg.conf.20080320034254
dexconf: error: cannot generate configuration file; 
xserver-xorg/config/monitor/horiz-sync not set.  Aborting.  Reconfigure the X 
server with "dpkg-reconfigure xserver-xorg" to correct this problem.
xserver-xorg postinst warning: error while preparing new Xorg X server
   configuration file in /etc/X11/xorg.conf.dpkg-new; not attempting to
   update existing configuration