ubuntu下延伸桌面

用了两个显示器，可以配置二者显示不同的分辨率。
ubuntu edgy, i915gm, 外接的显示器为VA902。
xorg.conf的配置为：

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/100dpi”
FontPath “/usr/share/X11/fonts/75dpi”
FontPath “/usr/share/fonts/X11/misc”
# path to defoma fonts
FontPath “/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType”
EndSection

Section “Module”
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” “pc104”
Option “XkbLayout” “us”
EndSection

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

Section “InputDevice”
Identifier “Synaptics Touchpad”
Driver “synaptics”
Option “SendCoreEvents” “true”
Option “Device” “/dev/psaux”
Option “Protocol” “auto-dev”
Option “HorizScrollDelta” “0”
EndSection

Section “InputDevice”
Driver “wacom”
Identifier “stylus”
Option “Device” “/dev/wacom” # Change to
# /dev/input/event
# for USB
Option “Type” “stylus”
Option “ForceDevice” “ISDV4” # Tablet PC ONLY
EndSection

Section “InputDevice”
Driver “wacom”
Identifier “eraser”
Option “Device” “/dev/wacom” # Change to
# /dev/input/event
# for USB
Option “Type” “eraser”
Option “ForceDevice” “ISDV4” # Tablet PC ONLY
EndSection

Section “InputDevice”
Driver “wacom”
Identifier “cursor”
Option “Device” “/dev/wacom” # Change to
# /dev/input/event
# for USB
Option “Type” “cursor”
Option “ForceDevice” “ISDV4” # Tablet PC ONLY
EndSection

Section “Extensions”
Option “Composite” “Enable”
EndSection

Section “Device”
Identifier “Intel Corporation Mobile 915GM/GMS/910GML Express Graphics Controller”
Driver “i810”
BusID “PCI:0:2:0”
VideoRam 131072
Option “MonitorLayout” “CRT,LFP”
# Option “Clone” “true”
Option “DRI” “true”
Option “XAANoOffscreenPixmaps”
Screen 0
EndSection

Section “Device”
Identifier “915-External”
Driver “i810”
BusID “PCI:0:2:0”
Option “MonitorLayout” “CRT,LFP”
Screen 1
EndSection

Section “Monitor”
Identifier “Generic Monitor”
Option “DPMS”
EndSection

Section “Monitor”
Identifier “External”
Option “DPMS”
EndSection

Section “Screen”
Identifier “Default Screen”
Device “Intel Corporation Mobile 915GM/GMS/910GML Express Graphics Controller”
Monitor “Generic Monitor”
DefaultDepth 24
SubSection “Display”
Depth 1
Modes “1024×768”
EndSubSection
SubSection “Display”
Depth 4
Modes “1024×768”
EndSubSection
SubSection “Display”
Depth 8
Modes “1024×768”
EndSubSection
SubSection “Display”
Depth 15
Modes “1024×768”
EndSubSection
SubSection “Display”
Depth 16
Modes “1024×768”
EndSubSection
SubSection “Display”
Depth 24
Modes “1024×768”
EndSubSection
EndSection

Section “Screen”
Identifier “External Screen”
Device “915-External”
Monitor “External”
DefaultDepth 24
SubSection “Display”
Depth 24
Modes “1280×1024”
EndSubSection
EndSection

Section “ServerLayout”
Identifier “Default Layout”
Screen “Default Screen”
Screen “External Screen” RightOf “Default Screen”
InputDevice “Generic Keyboard”
InputDevice “Configured Mouse”
InputDevice “stylus” “SendCoreEvents”
InputDevice “cursor” “SendCoreEvents”
InputDevice “eraser” “SendCoreEvents”
InputDevice “Synaptics Touchpad”
EndSection

Section “DRI”
Mode 0666
EndSection
Section “ServerFlags”
Option “Xinerama” “true”
EndSection