I'm not sure I can tell you how I got 800X600 working. I do know that it is the only resolution that kuki detects as common between the internal and external monitors.
The first problem is that the external monitor is not detected as a widescreen monitor and only 3 vga (?) screen resolutions appear to be supported.
I played around with xrandr for awhile and found that I could not add any modes that weren't detected as supported on both of the monitors.
Then I set out to set up a virtual desktop on the AAO and add a resolution supported by the external monitor manually.
However, several attempts following several outdated guides on the web resulted in me editing xorg.conf in nano from the command line (X would not start) and no joy.
Then I discovered grandr, a gui for xrandr that supposedly allows managing settings for multiple screens.
Oddly, when grandr is running, the AAOs display settings are not visible from xfce-display-settings.
So, using grandr, I boot with the external monitor attached. Kuki either boots as 1024X600, or 800X600, depending on whether I had the external attached when I shut down.
I turn off the AAO monitor (there doesn't seem to be support for different resolutions on different screens).
I set the external monitor to "auto" in grandr, then untick the auto box and select one of the 3 available resolutions manually. I click apply. Sometimes, this works, but usually, the external displays 1024X600 in the upper left portion of the monitor. I repeat the auto... resolution... apply sequence and then it seems to take, although the last time I did this yesterday, I had to restart X, with the external attached and the AAO screen off.
The only negative side effect of this, other than the hassle, is that my grub boot screen is now permanently in 800X600 -- an known ubuntu 8.10+ bug.
So, I think what I want to do is 1. discover how to create a virtual desktop and then 2. manually enter the correct mode line for the external monitor in xorg.conf.
However, my xorg.conf doesn't appear to support either the Virtual option or a mode line. Obviously, this information is now being stored elsewhere by the *buntus >= 8.10 and being "refreshed" at each boot.
I'm stumped and have spent far more time on the problem than I should have.