the change has to be made in your X configuration file
try looking for it in /etc/X11/x.org.conf, or similar.
You will need to change it to something similar to:
Code:
...
Section "Screen"
...
SubSection "Display"
Depth xxxx
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
...
EndSection
...
Make a backup of your old file, in case you screw up.
Add '"1280x1024" "1024x768"' for every depth
restart X (or your whole computer to apply)
Your distribution may also provide a graphical X configuration tool.