This is what I did to get Xinerama working. I've got one laptop with a wide screen display and an external monitor with normal 4:3 aspect ratio.

Furthermore, I'm using the free, open source video drivers (for my nVIDIA card), so I cannot use their nice GUI for setting it up. On the other hand, this approach will work for all kinds of video cards, so read on!

Changes to the X configuration

You need to enter the full view area to your /etc/X11/xorg.conf, adding up the resolutions of the two screens, as reported by the xrandr -q command.

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    SubSection     "Display"
        Depth       24
        #  1440+1600  900+1200
        Virtual     3040 2100

Using xrandr to set make use of the two screen

This sets up the external screen to the right of my laptop screen, positioning on the pixel by giving the resolution of the laptop screen.

$ xrandr  --output VGA1 --right-of LVDS --pos 1440x900 --auto

When I disconnect the external screen, I do the following to get all the windows on the external screen onto the laptop screen:

$ xrandr  --output VGA1 --off

Of course, I've added these lines to my .xinitrc to set this up automatically when I start my computer :-)