Problems with Dual Display with Radeon Mobility 7500

[William M. Quarles]

I'm trying to set up a spanning desktop on my Dell Latitude C640 laptop, 
which has an ATI Radeon Mobility 7500 graphics chipset and a Dell 1504FP 
LCD monitor in analog (VGA) mode. I use Gnome on Fedora 10.

I can (sometimes) get the spanning desktop to work using 
System/Preferences/Hardware/Screen Resolution from the Gnome menu with 
the laptop configured to be on the left, and the external display 
configured to be on the right. However, this is actually the opposite of 
my physical setup at my desk. Every time that I try to switch the 
displays to their actual layout, the desktop still spans, but the 
displays are mirrored, which is not only useless, but can also cause 
problems, especially when sometimes it results in the display without 
the top and bottom bars missing (!). Does anybody know how I can get 
this work properly with the laptop on the right and the external monitor 
on the left?

Thanks,
William