I believe you should be OK with that configuration, could be something with your CD-ROM Drive?

Hardware compatibility list is available at Redhat 9.0 HCL

You might try creating the boot floppy to start the install process - I've had some luck with that on previous versions of RH that didn't want to install from CD Boot. (Edit - just realized you already ruled this out. Doh!)

Please let us know how you make out, I've got an old P100 that's running a dusty old version of RH or Slackware and I did have it in the back of my mind to maybe try and throw RH 9 on it.

Good Luck