The extra entry is your new kernel. It doesn't hurt to keep the old kernel entry. Be sure to use the new one for a while to make sure it works before trying to remove the old one.
The entries are controlled by grub.conf, which should be in /boot/grub/ . Open it up and take a look at the listings. Find the one that lists your old kernel. It'll look something like...
title Red Hat Linux (2.4.18-18.8.0)
kernel /vmlinuz-2.4.18-18.8.0 ro root=LABEL=/ hdc=ide-scsi
The easiest thing to do is remark out the four lines by adding a # at the front of the line.
But, that doesn't remove the files. There are three files in your /boot/ for each kernel... vmlinuz-2.4.whatever, which is the actual kernel; initrd-2.4.whatever which is the INITial RamDisk; and System.map-2.4.whatever. Delete the entry in grub.conf, and the corresponding three files in /boot/ , and it will be gone.