My struggle for the perfect dual-monitor setup with my laptop and an external monitor finally came to an end.
I always wanted the following features to work:

• Big Desktop Setup (two Screens, laid out horizontally)
• Different Resolutions (I have a non-widescreen external monitor and a widescreen laptop screen, so i wanted
  the Resolutions to be 1280×800(laptop) and 1280×1024 (external) in this case.

This is how i did it:

1. Edit /etc/X11/xorg.conf
2. Put the following lines in bold into your device section:
   Section “Device”
   Identifier  “aticonfig-Device[0]”
   Driver      “fglrx”
   Option “DesktopSetup” “horizontal”
   Option “Mode2″ “1280×1024″
   Option “PairModes” “1280×800+1280×1024″
   Option “Capabilities” “0×00000800″
   EndSection

Explanation:

• DesktopSetup “horizontal” means your Screens are laid out horizontally. Another Option would be “vertical” if your
  Screens are on top of each other
• Option “Mode2″: This is the Resolution of the secondary Monitor (the external one in this case)
• Option “PairModes”: This line describes the valid Resolution pairs for both monitors
• Option “Capabilities”: Don’t know yet what this is, please tell me

If you insert the bold lines into your xorg.conf, you should get the desired Resolutions after restarting XOrg.
Unfortunately, this configuration has a small bug: The mouse cursor can be moved below the primary screen. Anyone know how 2 fix?