Problème changer la résolution linux (Virtual

Bonjour, j'ai installé hier VirtualBox et Ubuntu.

J'ai donc Ubuntu en machine virtuel qui marche correctement.

Mais voila le mode plein écran de VirtualBox laisse beaucoup de barres noir très désagréable ...

J'ai correctement installé L'Aditions et j'ai accées à un fichier xorg.conf.

Mais je ne sais que faire avec ce fichier voici son contenue : # 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"
EndSection

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

Merci d'avance de maider.
PS: Je suis sous vista.