unable to run gsynaptics on lenny

[vishnu vardhan <vishnuvardhan.linux@gmail.com>]

os : lenny 5.0.4

i have run gsynaptics from command line :
$ gsynaptics

and it has shown the following error :

GSynaptics couldn't initialize.
You have to set 'SHMConfig' 'true' in xorg.conf or XF86Config to use GSynaptics

i have edited the file of /etc/X11/xorg.conf and added support for synaptics. 

Section    "InputClass"
        Identifier    "enable synaptics SHMConfig"
        MatchIsTouchpad    "on"
        MatchDevicePath    "/dev/input/event*"
        Driver        "synaptics"
        Option        "SHMConfig"    "on"
EndSection

but, when i try to run # dpkg-reconfigure -phigh xserver-xorg, i am receiving the following error :

xserver-xorg postinst warning: overwriting possibly-customised configuration file; backup in /etc/X11/xorg.conf.20100928233119

how should i overcome the above error.