[Wylug-help] Desktop Spanning Multiple Screens

[Smylers]

I wish to use my laptop with an external monitor in addition to its
built-in screen, with the desktop spanning the screens.  I think this is
possible only if I have the screens virtually stacked (such that the
mouse pointer moves off the bottom of one and appears at the top of the
screen to the side of it); I'd been hoping to have them side by side.

But I'm slightly out of my depth in this stuff, so would appreciate
confirmation or suggestions of other things to try.

I'm running Ubuntu 9.04, with Gnome 2.26.1.  In 'Display Preferences'
some configurations with an unmirrored external monitor cause it to
prompt for root privs to change some system setting, which so far as I
can tell is simply tweaking xorg.conf to have SubSection "DIsplay" with
the Virtual setting, as described here:

  http://209.85.229.132/search?q=cache:http://intellinuxgraphics.org/dualhead.html&strip=1

That page includes the comment:

  # There is a known issue that DRI doesn't work on pre-965 if maximum
  # is larger than 2048x2048.

Indeed configurations with my 1280×800 laptop next to a 1024×768 monitor
(2304 pixels wide) cause nothing meaningful to display the next time I
log into X.  Configuring my laptop as only 1024×768 (so it has black
margins) works, as does arranging the displays vertically.

So I'm guessing that that limit applies to me and there's nothing I can
do about it -- more specifically that 'DRI' is some technology necessary
for what I'm trying to do, and that '965' is a chip number and it's
newer than the 'Mobile 945GM/GMS, 943/940GML Express Integrated Graphics
Controller' which lshw reports this laptop as having?

Thanks.

Smylers