You can delete the entries like
title Fedora Core (2.6.10-1.770_FC3) root (hd0,6) kernel /vmlinuz-2.6.10-1.770_FC3 ro root=LABEL=/12 initrd /initrd-2.6.10-1.770_FC3.img
from your grub.conf and leave only the first entry that corresponds to the newer kernel installation.
This is done automatically when you do: # rpm -e kernel-2.6.10-1.770_FC3 to remove the old kernel.
Paul.