OKAY, I've got dual displays working on my dell 600m with Mobility Radeon 9000 64mb with Ubuntu 7.10 Gutsy (caveat: no video playback regardless of file type, hrrmmmm....)

After deleting my WinXP partition (FREEDOM!) I decided to try Ubuntu 7.10 in that space, and see if I could get dual displays working. And low and behold, I did it. So, I deleted that partition, and dedicated the space on my hard drive to Ubuntu, the current working installation being 6.06 LTS, in dual display glory.
Note: I have a separate partition for my /home directory, and my whole hard drive backed up...

In an effort to keep all my programs and settings, I decided to UPGRADE from 6.06 up to 7.10, making all stops (6.06->6.10->7.04->7.10).

This worked great up until I tried to upgrade from 7.04 to 7.10. Even my dual display setup worked great in 7.04. However, trying to upgrade to 7.10 left me with a laptop that would not boot into a usable desktop, and a buggy command line, which was not useful to me for trouble shooting (when it worked at all)

***step one
- upgrade from 6.06 -> 6.10
gksu "update-manager -c"
This tells the System update manager to look for a full upgrade, and not just and update.

->7.04
(menu)System->Administration->Update Manager
My dual monitor setup from 6.06 LTS still works here, yeah! If you've come this far, stay put!

->7.10 STOP!!!
(menu)System->Administration->Update Manager
WAIT, nope, system will not fully boot after upgrading from 7.04 to 7.10.
Crap.

***Clean 7.10 install
Before installing 7.10, power off and disconnect external display...
With my /home directory on a second partition, I only had to reinstall thunder-bird, all email and account settings present at first run, yeah!)

***from here on it is info I found here - http://www.thinkwiki.org/wiki/Xorg_RandR_1.2

***step two - clean up the xorg.conf file (restart x after each change to ensure it still works...) see http://www.thinkwiki.org/wiki/Xorg_RandR_1.2 for details. The only change I had to make to my conf file was this, add the following line so the "Display" subsection of "Screen" looks something like this:

SubSection "Display"
Depth 24
Modes "1600x1200" "1280x1024" "1024x768" "800x600" "640x480"
# ADD A VIRTUAL LINE TO PROVIDE FOR THE LARGEST SCREENS YOU WILL HOTPLUG
Virtual 2048 2048
EndSubSection


***step three - get xrandr working for you

in a terminal:

xrandr -q

This should give you an idea of what your current display is doing,
Now, plug in and power up your external display.

xrandr -q

This should now show your external display, in my case, VGA-0 at 1280x1024

After going over all the info at the ThinkWiki (link above), I found this command got my displays working together like I want (dual display, external right of main, which is LVDS in my case)

xrandr --output VGA-0 --mode 1280x1024 --pos 1024x0

NOTE: this command should take effect immediately, no need to restart x.
EXPLANATION: "output" is which display you're commanding, "mode" is the resolution you're telling that display to show, and "pos" is where desktop starts on that display, in my case my main display (laptop) has a res of 1024x768, so I want the external display to line up at the top "0" but start "1024" to the right.

***step four - get xrandr working at boot time (well, after log-in anyway)

create a text document, should look like this:

#!/bin/sh
xrandr --output VGA-0 --mode 1280x1024 --pos 1024x0


save it (I saved mine as xorg_710_bigdesktopscript in my /home/username directory) , then in nautilus, right click to get the properties and set it to "Execute- allow executing file as program" under the Permissions tab.

Now go to menu System->Preferences->Sessions and Add your new file.

Now restart x, press ctrl-alt-backspace

it should look like your main display is being mirrored, however, this should
change once you are logged in. If not, check your files for typos...