After spending a day and a night I have got this working. Not perfectly as restarting Xserver by CTRL+ATL+BACKSPACE would show a scrumbled screen (see below for a fix).
This is the hardware configuration I tried to get working:
The Laptop “Dell Latitude D810″ ist connected to a docking station “D/Port Advanced Port Replicator”, the external monitor is connected to the DVI-D Port of the docking station. I wanted a big desktop stretched over both monitors, laptop and external, that makes sense cause the laptop can be fixed in a upright position by the D/View computer stand, the resolution of both monitors is 1680×1050.

The main steps were:

  1. Updating the fresh installed system.
  2. Removing the xorg-driver-fglrx driver package if installed.
  3. Downloading the latest ati fglrx driver from http://ati.amd.com/support/drivers/linux/linux-radeon.html and installing them by just running the downloaded ati-driver-installer-8-11-x86.x86_64.run as root.
  4. Reading Unofficial Ati Linux Driver Wiki
  5. Following descriptions in Dual Monitor Support With Binary, ATI-Only Big-Desktop
  6. Running aticonfig according to the descriptions:
    root@myhome:# aticonfig --initial --overlay-type=Xv
root@myhome:# aticonfig --desktop-setup=horizontal --sync-vsync=on --add-pairmode=Width0xHeight0+Width1xHeight1
  7. Tuning my xorg.conf by adding some options to the device section.

To make this descriptions complete here is my xorg.conf :

# xorg.conf (X.Org X Window System server configuration file)

Section "ServerLayout"
  Identifier     "Default Layout"
  Screen      0  "aticonfig-Screen[0]-0" 0 0
  InputDevice    "Synaptics Touchpad"
EndSection

Section "Files"
EndSection

Section "Module"
EndSection

Section "InputDevice"
  Identifier  "Generic Keyboard"
  Driver      "kbd"
  Option      "XkbRules" "xorg"
  Option      "XkbModel" "pc105"
  Option      "XkbLayout" "de"
  Option      "XkbOptions" "lv3:ralt_switch"
EndSection

Section "InputDevice"
  Identifier  "Configured Mouse"
  Driver      "mouse"
  Option      "CorePointer"
EndSection

Section "InputDevice"
  Identifier  "Synaptics Touchpad"
  Driver      "synaptics"
  Option      "SendCoreEvents" "true"
  Option      "Device" "/dev/psaux"
  Option      "Protocol" "auto-dev"
  Option      "HorizEdgeScroll" "0"
EndSection

Section "Monitor"
  Identifier   "Configured Monitor"
EndSection

Section "Monitor"
  Identifier   "aticonfig-Monitor[0]-0"
  Option      "VendorName" "ATI Proprietary Driver"
  Option      "ModelName" "Generic Autodetecting Monitor"
  Option      "DPMS" "true"
EndSection

Section "Device"
  Identifier  "Configured Video Device"
EndSection

Section "Device"
  Identifier  "aticonfig-Device[0]-0"
  Driver      "fglrx"
  Option      "VideoOverlay" "on"
  Option      "OpenGLOverlay" "off"
  Option   "Mode2"         "1680x1050" #Resolution for second monitor
  Option      "DesktopSetup" "horizontal"  #Enable Big Desktop
  Option "DesktopSetup" "LVDS,AUTO" #the types of monitors that is connected LVDS = LCD, CRT, AUTO
       Option "EnablePrivateBackZ" "yes" #Enable 3d support <= May Not Work
       Option "HSync2" "65" #This sets the horizontal sync for the secondary display. 
       Option "VRefresh2" "60" #This sets the refresh rate of the secondary display.
  Option      "Capabilities" "0x00000800"
  Option      "PairModes" "0x0+0x0"
  Option      "EnableMonitor" "lvds,tmds1" # may be you dont need this
  BusID       "PCI:1:0:0"
EndSection

Section "Screen"
  Identifier "Default Screen"
  Device     "Configured Video Device"
  Monitor    "Configured Monitor"
EndSection

Section "Screen"
  Identifier "aticonfig-Screen[0]-0"
  Device     "aticonfig-Device[0]-0"
  Monitor    "aticonfig-Monitor[0]-0"
  DefaultDepth     24
  SubSection "Display"
    Viewport   0 0
    Depth     24
  EndSubSection
EndSection

First I could fix the freezing of X on logout by disabling acpi. But then I found at Wiki Troubleshooting some hints and had success with:
File: /etc/kde3/kdm/kdmrc
TerminateServer=true
didn’t work alone. Added
sudo /usr/sbin/update-rc.d -f atieventsd remove
with success even with acpi enabled.

Grub menu disabling acpi:

# set this in /boot/grub/menu.lst
title           Ubuntu 8.04.1, kernel 2.6.24-21-generic ACPI=off
root            (hd0,5)
kernel          /boot/vmlinuz-2.6.24-21-generic root=UUID=45c3fc6b-bbcb-4320-8470-9228582f3115 ro quiet pci=noacpi acpi=off
initrd          /boot/initrd.img-2.6.24-21-generic