I've just bought a
new monitor, whose maximum resolution is 1440x900. If I plug it in to my ThinkPad T40's dock and boot, X picks it up and applies the correct resolution. However, the image is a duplicate of the image on the laptop's screen. I've managed to set all the panels to fit the external monitor, so it all looks OK, except if I maximise a window, it maximises to 1024x768 (the laptop's resolution), not to 1440x900 (the external monitor resolution).
I've been googling, and people mention xrandr a lot with respect to configuring dual screen. So I tried all the relevant commands posted on the ThinkWiki article, for example, but I can't get an extended desktop, that is where I can scroll from the edge of one monitor into the appropriate edge of the next.
I'm aware of Xinerama, whose job it is to do this sort of thing, but according to wikipedia (source of all knowledge!

) the project is being phased out in favour of xrandr. However, I can't find anything on google about how to configure an extended desktop.
Does anyone have any experience with Xinerama? Is that even the appropriate tool? Or can this be done with xrandr?
Thanks.
Mods: I was torn between the Hardware and Software forums, but the monitor works, so the problem is software based, as far as I can determine.