I have Fedora 8 on this server..to install webmin:

yum -y install perl-Net-SSLeay
rpm -ihv http://superb-west.dl.sourceforge.net/sourceforge/webadmin/webmin-1.480-1.noarch.rpm

Then login to the Webmin interface: https://site.com:10000/ and enable Networking -> Bandwidth Monitoring.
It uses iptables and reads counters for IN/OUT. A good practice would be to change the port. This should work on Centos 5.2 / Centos 5.3 too. Also keep in mind that default installations of Fedora or Centos might require to open port 10000. On this Fedora 8 I had to :

iptables -I RH-Firewall-1-INPUT -p tcp -m state –state NEW -m tcp –dport 10000 -j ACCEPT
service iptables save