Dual Monitor configuration help

Hi,

I have ubuntu 7.10 running in a dual boot configuration.
So everything I want to achieve does already function under winXP on the same machine, thus we can exclude any hardware handicaps concerning my goal, to begin with.

I have two LCD displays:
#1 DELL 2407WFP
#2 ViewSonic VP211b

My graphic adapter is:
ATI Technologies Inc R480 [Radeon X850XT Platinum (PCIE)]

I was already able to have my DELL display running under its native resolution of 1920x1200. But only when I disconnected the second monitor.

I was applying the following hints to achieve a dual display configuration. http://gentoo-wiki.com/HOWTO_Dual_Monitors

Here is my xorg.conf file:
# xorg.conf (xorg X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the xorg.conf manual page.
# (Type "man xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following command:
# sudo dpkg-reconfigure -phigh xserver-xorg

Section "Files"
EndSection
Section "Module"
    Load "dbe" # Double-Buffering Extension
    Load "v4l" # Video for Linux
    Load "extmod"
    Load "type1"
    Load "freetype"
    Load "glx"
    Load "dri"
EndSection
Section "InputDevice"
 Identifier "Generic Keyboard"
 Driver "kbd"
 Option "CoreKeyboard"
 Option "XkbRules" "xorg"
 Option "XkbModel" "pc105"
 Option "XkbLayout" "de"
EndSection

Section "InputDevice"
 Identifier "Configured Mouse"
 Driver "mouse"
 Option "CorePointer"
 Option "Device" "/dev/input/mice"
 Option "Protocol" "ImPS/2"
 Option "ZAxisMapping" "4 5"
 Option "Emulate3Buttons" "true"
EndSection

Section "InputDevice"
 Driver "wacom"
 Identifier "stylus"
 Option "Device" "/dev/input/wacom"
 Option "Type" "stylus"
 Option "ForceDevice" "ISDV4" # Tablet PC ONLY
EndSection

Section "InputDevice"
 Driver "wacom"
 Identifier "eraser"
 Option "Device" "/dev/input/wacom"
 Option "Type" "eraser"
 Option "ForceDevice" "ISDV4" # Tablet PC ONLY
EndSection

Section "InputDevice"
 Driver "wacom"
 Identifier "cursor"
 Option "Device" "/dev/input/wacom"
 Option "Type" "cursor"
 Option "ForceDevice" "ISDV4" # Tablet PC ONLY
EndSection

Section "Device"
 Identifier "ATI Technologies Inc R480 [Radeon X850XT Platinum (PCIE)]"
 Driver "ati"
 BusID "PCI:1:0:0"
 Option "DynamicClocks" "on" # optional
        Option "MergedFB" "true"
        Option "CRT2Position" "LeftOf"
EndSection

Section "Monitor"
 Identifier "DELL 2407WFP"
 Option "DPMS"
 HorizSync 30-83
 VertRefresh 56-76
EndSection

Section "Screen"
 Identifier "Default Screen"
 Device "ATI Technologies Inc R480 [Radeon X850XT Platinum (PCIE)]"
 Monitor "DELL 2407WFP"
 DefaultDepth 24
 SubSection "Display"
  Depth 24
  Modes "1920x1200"
  Virtual 3520 1200 #The first value is the sum of horizontal resolutions your displays use
 EndSubSection
EndSection

Section "ServerLayout"
 Identifier "Default Layout"
 Screen "Default Screen"
 InputDevice "Generic Keyboard"
 InputDevice "Configured Mouse"

# Uncomment if you have a wacom tablet
# InputDevice "stylus" "SendCoreEvents"
# InputDevice "cursor" "SendCoreEvents"
# InputDevice "eraser" "SendCoreEvents"
EndSection

Having that configuration boots into a twin display mode in which each display runs under 1600x1200. That is native for the second monitor but not for the first one.

Anyway, after login I applied the following command under root:
 xrandr --output VGA-0 --auto --right-of DVI-0 --auto

The result is on large desktop BUT with only a virtual resolution of 3200x1200 and not as specified 3520x1200.
See output:
xrandr
Screen 0: minimum 320 x 200, current 3200 x 1200, maximum 3520 x 1200
VGA-0 connected 1600x1200+1600+0 (normal left inverted right) 432mm x 324mm
   1600x1200 60.0*+ 59.9
   1280x1024 75.0
   1152x864 75.0 74.8
   1024x768 75.1 70.1 60.0
   832x624 74.6
   800x600 72.2 75.0 60.3 56.2
   640x480 75.0 72.8 66.7 60.0
   720x400 70.1
S-video disconnected (normal left inverted right)
DVI-0 connected 1600x1200+0+0 (normal left inverted right) 519mm x 324mm
   1920x1200 60.0 +
   1600x1200 59.9*
   1680x1050 60.0
   1280x1024 75.0 59.9
   1152x864 74.8
   1024x768 75.1 60.0
   800x600 75.0 60.3
   640x480 75.0 60.0
   720x400 70.1

The advantage of that approach is, that I can work with the free ATI driver and keep all 3D acceleration and desktop effects.

Any Idea how I can get the first DELL monitor running under its native resolution of 1920x1200?