you need to add the driver to the initrd.img file..

if you boot from the rescue CD mode, and mount the root filesystem, (and other filesystems necessary to make this trick work)

then you will need to run thi s command (chroot'd of course)

/sbin/mkinitrd /boot/initrd-2.4.21-20.ELsmp.img 2.4.21-20.ELsmp.img

this of course assumes you are running 2.4.21-20.ELsmp

You can always add it by hand as well.
cp /boot/initrd.FOO /tmp/initrd.gz
mkdir /mnt/loop
mount -o loop /tmp/initrd /mnt/loop

much around with the files in that, umount, gzip, copy and boot

I've had to do this lots of times for oddball systems like that.

You may also want to look into kernel-unsupported RPM packages, stupid redhat decided that anything that wasn't tested to their standards would go in a separate RPM package. usefull things like SGI XFS, and IBM JFS, and reiserfs.. *FUME*