Hand edit the file /etc/X11/XF86Config-4 file and change the info in the Section "Monitor". Make a backup incase you mess up and restartx.
# Configuring X
Linux Hardware Compatibility HOWTO - video cards
The Linux XFree86 HOWTO
Configuring XFree86 for a Non-Specific Linux Distribution
Common X configuring tools:
Debian - dpkg-reconfigure xserver-xfree86
Mandrake - XFdrake
Redhat 7.3 down - the
setup utility leads to several config tools
Redhat 7.3 up - redhat-config-xfree86
You may have these tools:
XF86Setup
XFree86 -configure
Xconfigurator
xf86cfg
xf86config
xconf
ATI Linux drivers
If you need the
nVidia driver, you want the Linux IA32 driver