LinuxQuestions.org - Changing resolution in X

open up /etc/X11/xorg.conf

you will see a section that looks somewhat similiar to this :

Code:

Section "Screen"

    Identifier  "Screen 1"

    Device      "Standard VGA"

    Monitor    "My Monitor"

    DefaultDepth 16



    Subsection "Display"

        Depth      8

        Modes      "1280x1024" "1024x768" "800x600" "640x480"

        ViewPort    0 0

    EndSubsection

    Subsection "Display"

        Depth      16

        Modes      "1280x1024" "1024x768"

        ViewPort    0 0

    EndSubsection

    Subsection "Display"

        Depth      24

        Modes      "1280x1024" "1024x768" "800x600" "640x480"

        ViewPort    0 0

    EndSubsection

EndSection

now as you can see towards the top the "default depth" is "16", so the
only section out of all that, that will be used is the following because
as you see the depth is 16 for this section and that is default :

Code:

Subsection "Display"

        Depth      16

        Modes      "1280x1024" "1024x768"

        ViewPort    0 0

    EndSubsection

now when you look at the modes line you see that the first one
listed is 1280x1024, meaning that this is the resolution it will
load up to... so if you want 640x480 to be used, then put it
as the first option in modes ...