For NVidia Drivers: http://doc.gwos.org/index.php/Latest_Nvidia_Dapper

First, just in case, make a back up of you current file:

sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.org

sudo cp /etc/X11/xorg.conf.org /etc/X11/xorg.conf

Step 1.

First, try a quick fix. It may not work, but if is does you are done.

sudo dpkg-reconfigure -phigh xserver-xorg

• The -phigh option selects the defaults for everything but the screen resolution.
• To select a resolution, use the arrow keys to move up and down, hit the "Space bar" to select/unselect a resolution.
• You will recieve a message indication your old file was backed up.

If that fails you will need to know the specifications of your monitor and video card.

Step 2.

Monitor information
Google search your monitor by make and model. You are looking for the technical specifications for the horizontal and vertical refresh rates.

Videocard information In a terminal run this command:

lspci | grep VGA

If you have Gnome running:

sudo gedit /etc/X11/xorg.conf

sudo nano /etc/X11/xorg.conf

You are looking for the following sections:

Monitor:

Section "Monitor"Identifier "Generic Monitor"HorizSync 30.0 - 70.0VertRefresh 50.0 - 160.0Option "DPMS"EndSection

1. Reduce the DefaultDepth (this is color depth, 16 is more then adequate for most people and 24 may be excessive).
   Change it to 16:
   
   Code:

   DefaultDepth 16    Subsection "Display"        Depth       16        Modes "1600x1200" "1280x1024" "1024x768" "800x600" "640x480"    EndSubsection

2. Make sure the Subsection "Display" lists your desired resolution.
   On occasion I have "fooled" X into giving me my desired resolution by setting the resolution 1 step higher.
   If you are not sure, you can use "1600x1200" and reduce it later.