after edit xorg.config file error

my original file was like this

# xorg.conf (X.Org 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.
#
# Note that some configuration settings that could be done previously
# in this file, now are automatically configured by the server and settings
# here are ignored.
#
# 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 "Device"
 Identifier "Configured Video Device"
EndSection

Section "Monitor"
 Identifier "Configured Monitor"
EndSection

Section "Screen"
 Identifier "Default Screen"
EndSection

EndSection

But after xfce4.6 installation and when using it display didn't gave a good resolution(800*600)
so I edited the xorg.config as below

# xorg.conf (X.Org 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.
#
# Note that some configuration settings that could be done previously
# in this file, now are automatically configured by the server and settings
# here are ignored.
#
# 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 "Device"
 Identifier "Configured Video Device"
EndSection

Section "Monitor"
 Identifier "Configured Monitor"
EndSection

Section "Screen"
 Identifier "Default Screen"
 Monitor "Configured Monitor"
 Device "Configured Video Device"
 Section "Screen"
 Identifier "Default Screen"
 Monitor "Configured Monitor"
 Device "Configured Video Device"
 Defaultdepth 24
 SubSection "Display"
 Depth 24
 Modes "1024x768_75" "800x600_85"
 EndSubSection
EndSection

EndSection

now it gives me resolution 1024*768
but just after boot loader
it gives an error message

"System runs under low graphic mode"
and allow us to edit xorg.config , to use default etc.
but when i cancel and
use to law graphic mode only for this session it takes
1 minute to start
although system tells that it is low graphic mode it is not bad and gives 1024*768 resolution which is sufficient to me.

I need to disable error message and dialog which offers to edit and configure xorg.config file and need quick boot with out 1 minute delay.
thank you.