Log in as root and command:
XFdrake
You didn't mention what you are using for a video card, so all we can do is post some links if that doesn't work for you.
# Configuring X
Linux Hardware Compatibility HOWTO - video cards
The Linux XFree86 HOWTO
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
nVidia Driver Installation NHF
nVidia driver page Most likely, you want the Linux IA32 driver