forçar la resolució de pantalla ubuntu

editant fitxer /etc/X11/xorg.conf:

Section "Device"
    Identifier    "Configured Video Device"
EndSection

Section "Monitor"
    Identifier    "Configured Monitor"
    VendorName     "Unknown"
    ModelName      "Samsung SyncMaster"
    HorizSync       30.0 - 61.0
    VertRefresh     56.0 - 75.0
    Option         "DPMS"
EndSection

Section "Screen"
    Identifier    "Default Screen"
    Monitor        "Configured Monitor"
    Device        "Configured Video Device"
    DefaultDepth    24
    SubSection "Display"
        Modes        "1024x768" "800x600" "640x480"
    EndSubSection
EndSection