The xfce4 desktop resolution program simply uses 'xrandr' to query the server for supported resolutions.
Some newer drivers can query the monitor to determine what resolutions are supported but older ones need to be configured manually in your xorg.conf file. If you attach your /etc/X11/xorg.conf and /var/log/Xorg.0.log files to a post here, we might be able to point you in the right direction.
Adam
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