I’ve just got my ATI Technologies Inc RV280 [Radeon 9200 SE] graphics card working under a dual-head set up under Ubuntu Breezy. I had a working configuration under Etch, but I’m pleased to find out that the Ubuntu configuration is actually slightly cleaner. It uses the open source “radeon” driver, rather than the propriatory “fglrx” driver. Using a dual-head setup means that 3D acceleration doesn’t work, but I believe that is also the case for the “fglrx” driver. 3D acceleration did work under the “radeon” driver before switching to a dual-head mode.
Being GNOME rather than XFCE, enabling the Xinerama extension means that I can move windows from one panel to the other and not have them maximise across both panels. (Most window managers aren’t aware of things like “middle edge” detection under Xinerama and treat the whole display as one unbroken space.) I only have the “window dressing” (menu, application bars) on the first screen. At least that’s the default arrangement - panels can be moved to other locations on either panel by dragging. Maximising a window causes it to fill just one panel, not both. I’m happy with this arrangement. Commenting the Xinerama options out means that I have a separate set of window dressing on each monitor but can’t move windows between the two.
The end result looks like this:

For the record, here’s my xorg.conf:
# /etc/X11/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 /etc/X11/xorg.conf manual page.
# (Type “man /etc/X11/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”
FontPath “/usr/share/X11/fonts/misc”
FontPath “/usr/share/X11/fonts/cyrillic”
FontPath “/usr/share/X11/fonts/100dpi/:unscaled”
FontPath “/usr/share/X11/fonts/75dpi/:unscaled”
FontPath “/usr/share/X11/fonts/Type1″
FontPath “/usr/share/X11/fonts/CID”
FontPath “/usr/share/X11/fonts/100dpi”
FontPath “/usr/share/X11/fonts/75dpi”
# paths to defoma fonts
FontPath “/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType”
FontPath “/var/lib/defoma/x-ttcidfont-conf.d/dirs/CID”
EndSection
Section “Module”
Load “GLcore”
Load “i2c”
Load “bitmap”
Load “ddc”
Load “dri”
Load “extmod”
Load “freetype”
Load “glx”
Load “int10″
Load “type1″
Load “vbe”
EndSection
Section “InputDevice”
Identifier “Generic Keyboard”
Driver “kbd”
Option “CoreKeyboard”
Option “XkbRules” “xorg”
Option “XkbModel” “pc105″
Option “XkbLayout” “gb”
EndSection
Section “InputDevice”
Identifier “Configured Mouse”
Driver “mouse”
Option “CorePointer”
Option “Device” “/dev/input/mice”
Option “Protocol” “ImPS/2″
Option “Emulate3Buttons” “true”
Option “ZAxisMapping” “4 5″
EndSection
Section “Device”
Identifier “DVIport”
BoardName “ATI Technologies, Inc. Radeon 9200 SE (RV280)”
Driver “radeon”
BusID “PCI:1:0:0″
Screen 0
EndSection
Section “Device”
Identifier “VGAport”
BoardName “ATI Technologies, Inc. Radeon 9200 SE (RV280)”
Driver “radeon”
BusID “PCI:1:0:0″
Screen 1
EndSection
Section “Monitor”
Identifier “SyncMasterL”
Option “DPMS”
EndSection
Section “Monitor”
Identifier “SyncMasterR”
Option “DPMS”
EndSection
Section “Screen”
Identifier “VGAScreen”
Device “VGAport”
Monitor “SyncMasterR”
DefaultDepth 24
SubSection “Display”
Depth 1
Modes “1280×1024″ “1152×864″ “1024×768″ “832×624″ “800×600″ “720×400″ “640×480″
EndSubSection
SubSection “Display”
Depth 4
Modes “1280×1024″ “1152×864″ “1024×768″ “832×624″ “800×600″ “720×400″ “640×480″
EndSubSection
SubSection “Display”
Depth 8
Modes “1280×1024″ “1152×864″ “1024×768″ “832×624″ “800×600″ “720×400″ “640×480″
EndSubSection
SubSection “Display”
Depth 15
Modes “1280×1024″ “1152×864″ “1024×768″ “832×624″ “800×600″ “720×400″ “640×480″
EndSubSection
SubSection “Display”
Depth 16
Modes “1280×1024″ “1152×864″ “1024×768″ “832×624″ “800×600″ “720×400″ “640×480″
EndSubSection
SubSection “Display”
Depth 24
Modes “1280×1024″ “1152×864″ “1024×768″ “832×624″ “800×600″ “720×400″ “640×480″
EndSubSection
EndSection
Section “Screen”
Identifier “DVIScreen”
Device “DVIport”
Monitor “SyncMasterL”
DefaultDepth 24
SubSection “Display”
Depth 1
Modes “1280×1024″ “1152×864″ “1024×768″ “832×624″ “800×600″ “720×400″ “640×480″
EndSubSection
SubSection “Display”
Depth 4
Modes “1280×1024″ “1152×864″ “1024×768″ “832×624″ “800×600″ “720×400″ “640×480″
EndSubSection
SubSection “Display”
Depth 8
Modes “1280×1024″ “1152×864″ “1024×768″ “832×624″ “800×600″ “720×400″ “640×480″
EndSubSection
SubSection “Display”
Depth 15
Modes “1280×1024″ “1152×864″ “1024×768″ “832×624″ “800×600″ “720×400″ “640×480″
EndSubSection
SubSection “Display”
Depth 16
Modes “1280×1024″ “1152×864″ “1024×768″ “832×624″ “800×600″ “720×400″ “640×480″
EndSubSection
SubSection “Display”
Depth 24
Modes “1280×1024″ “1152×864″ “1024×768″ “832×624″ “800×600″ “720×400″ “640×480″
EndSubSection
EndSection
Section “ServerLayout”
Identifier “Default Layout”
Screen “VGAScreen”
Screen “DVIScreen” LeftOf “VGAScreen”
InputDevice “Generic Keyboard”
InputDevice “Configured Mouse”
Option “Xinerama” “on”
Option “Clone” “on”
EndSection
Section “DRI”
Mode 0666
EndSection