By default Debian Lenny and Ubuntus (even the latest 9.04 version) can only show 800x600 as the maximum resolution while the correct resolution should be 1024 x 768
To overcome the problem usually I copied the xorg.conf file from any other distros (mostly Mepis and Sidux).
Today I found the above references, and it turns out that the solution is really simple, just add 2 lines in the "Monitor" section of the default xorg.conf as follows:
(The above values I took from sidux's xorg.conf).
Now from the System > Preferences > Screen Resolution menu, I can choose the refresh rate (60, 70 or 75 Hz).
Below is the complete xorg.conf
# 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
# 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
Identifier "Generic Keyboard"
Option "XkbRules" "xorg"
Option "XkbModel" "pc104"
Option "XkbLayout" "us"
Identifier "Configured Mouse"
Identifier "Configured Video Device"
Identifier "Configured Monitor"
Identifier "Default Screen"
Monitor "Configured Monitor"