xrandr does work on Lenny.
We've got
two laptops using it with no
problems. Studying
http://www.thinkwiki.org/wiki/Xorg_RandR_1.2
was
really helpful, particularly understanding the 2048x2048 concept in the
Virtual screen section. With xrandr you keep your xorg.conf file to a
minimum.
Obviously your mileage may vary but if it's any help, my xrandr command
is:
$ xrandr --output LVDS --mode 1024x768 --output VGA-0 --mode 1024x768
--left-of LVDS.
I had to change VGA to VGA-0. You can find your own display types with:
$ xrandr -q
That's it, although in Xfce I have to log out and then back in again to
get the extra widths showing in the desktop workspaces.
Hope it helps
Maggie