[OT] What file can I use to change Ubuntu 9.10 display characteristics?

This question is vaguely related to R in that I have managed to wedge
the netbook computer that I use for demonstrations of R and I would
like to know how to un-wedge it so I can continue to give lectures
about R.

I have an ASUS eee PC running the netbook-remix version of Ubuntu
9.10.  The X server in 9.10 is a little different from earlier
versions in that it does not require an explicit configuration file
for the X server ( /etc/X11/xorg.conf, IIRC) as it can autoconfigure.
That it did fine.

When I attached the netbook to the VGA input for the video projector
in a lecture hall I found that it dropped the resolution to 640 x 480,
which is undesirably coarse.  I decided to tinker and see if I could
get it to 800 x 600 on both the built-in display and the video
projector.  I used the GUI application accessed through the menu
sequence System -> Preferences -> Display.

I believe that at one point I actually got things working that way
but, of course, I couldn't leave well-enough alone and decided to try
to get a resolution with 768 lines as I knew the projector supported
that.  That was when it all went south.  The system froze with a blank
screen and blank projector output.  I needed to do a hard power-down
and reboot without the projector plugged in.  Fortunately I didn't
lose the disk (I run ext2 file systems as a journaled file system puts
too much wear on the solid-state disk) and I can use the computer
without an external display.

Now I can use the netbook when it is not connected to an external VGA
display but as soon as I connect the external VGA it freezes,
presumably because it is trying to use an unsupported video mode.  The
information on the video modes I selected must be stored somewhere but
I can't find out where.  Does anyone know where that GUI application
caches its information?  All this GUI configuration is great unless
the thing you are trying to configure is the display and you can't get
that working.  In the old days when you needed to edit a text-based
configuration file you had a fighting chance of doing that even if you
could only get the barest of systems, a text console device, running.