To solved monitor resolutions 800x600 only

Some time ago I was preoccupied with the act of the monitor. for one thing, I tried to reformat my PC that was Ubuntu 9.04 to 10:04.

But the monitor resolution stuck on 800x600 size only.
I finally tried the brain with a little tweaking xorg.conf file reading in x.org , with steps below:
type in terminal :

~$ sudo apt-get install xorg*

than :

~$ sudo gedit /etc/X11/xorg.conf

add this :

HorizSync       28-73

and look like here :

# xorg.conf (X.Org X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the xorg.conf manual page.
# (Type "man xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# Note that some configuration settings that could be done previously
# in this file, now are automatically configured by the server and settings
# here are ignored.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following command:
#   sudo dpkg-reconfigure -phigh xserver-xorg

Section "Device"
        Identifier      "Configured Video Device"
EndSection

Section "Monitor"
        Identifier      "Configured Monitor"
        HorizSync       28-73
        EndSection

Section "Screen"
        Identifier "Default Screen"
        Monitor  "Configured Monitor"
        Device  "Configured Video Device"
EndSection

don't forget to restart GDM (Gnome Desktop Manager) :

$ sudo /etc/init.d/gdm restart