I have a Dell C640 laptop running dual headed with Xinerama.
My onboard display can only do 1024x768 but the external monitor can
do 1280x1024, so I would rather this be the 'primary' screen (i.e. the
one where the tool bars and various other widgets go).

I have played with swapping screens in xf86config, I have changed the
layout so Screen1 is LeftOf instead of RightOf Screen0 etc., I have
changed Screen1 to be the 'other' pci-based video card but every time
I start X my menubars (on Gnome based XD2) appear on the onboard

any hints?