You probably want to investigate something called ipsets which works hand in hand with iptables and is much more efficient. This is not currently available for CentOS 5 but I am about to try to get this into ELRepo. If you are comfortable with building your own RPMs then there is a set of SRPMs attached to an ELRepo ticket that I have modified to allow ipsets to be used on CentOS 5. Redhat are in the process of adding ipset support to RHEL 6.2 (it's in the beta). I do have a more recent build of the ipset SRPMs that contain init scripts for ipsets so that the contents can be saved/restored across reboots but there are no code changes to the set that are on ELrepo ticket 000083.
The guide you pointed to is rather more complicated than I was proposing. You would need to perform the step 1 from that but to rebuild previously built SRPMs is rather easier - you just run `rpm --install whatever.src.rpm` then `rpmbuild -bb whatever.spec` Then solve all the missing dependencies for packages that the build requires and repeat until it works!