How to set up an extended desktop on dual monitors in Kubuntu 9.04?

How to set up an extended desktop on dual monitors in Kubuntu 9.04?

I'm using my laptop (Lenovo Thinkpad T60) and I have a secondary monitor (which has a different resolution than my laptop screen) connected by VGA. Kubuntu recognizes the monitor and it functions but my desktop is mirrored (i.e. an identical copy) across the dual monitors (which is useless to me). How can I extend my desktop across both monitors?