Before you start, make sure you backup your original /etc/X11/xorg.conf !

I had quite an unpleasant experience trying to set up Laptop + External Displays with different resolutions in SuSE 10.0 GM.
My notebook is DELL Latitude D600 with mobile ATI 9200 (14" 1024x768 max. res.) + external 19" DELL 1905 FP display.
I could not force the correct resolution onto the second display via SAX, since Xinerama does not support different resolutions on multiple displays (as far as I know).
The max was the same resolution on both displays, with the parameters for the worse one (the laptop one), which was 1024x768 (nightmare on the 19").
What helped at the end was the ATI proprietary driver, which can be downloaded from the ATI site:

ati.com/support/driver.html --> Linux Drivers and Software --> your_model_ATI.run

Here it is important that you download the ATI Driver Installer "*.run" which is about 60 MBs big (at least the one I doownloaded)! I did not use the pre-compiled *.rpms available on the site.
Once you download it, run it as root in a shell.
I chose the "Install driver" (1st) option & was prompted to run "fglrxconfig" at the end.
When you run it, make sure you configure the Dual Monitors sector.
The script does everything for you, you just respond to the dialogues by pushing Enter.
Except for the Dual Monitors setup, I always chose default or what the script suggested.
At the end it asks if the existing xorg.conf should be replaced with the newly generated one, you should type "y" and hit Enter.

Restart your PC/laptop and enjoy!

great I've been looking for this for a long time.