I've never run into this one before; what type of graphics card is it?
About the only thing I could find on Google with any sort of valid advice was this:
http://www.libranetlinux.com/librane.../msg00201.html
This basically outlines that you have no /dev/agpgart device in order for X to connect with, and must create the device node. I'd be surprised if the install you did neglected to leave this device file out, but you never know.
I always configure X like this:
[SHELL]
prompt# XFree86 -configure
[/SHELL]
(let X autoprobe your devices)
Add some modelines to the output file (/root/XF86Config.new) and test it with
[SHELL]
prompt# XFree86 -xf86config XF86config.new
[/SHELL]