CentOS

I just upgraded my VPS to a dedicated server, but now I don't know how to keep track of my monthly bandwidth usage. Does anyone know of a tool in CentOS 5 or something I can install to monitor this? I know iptables can show the amount of network traffic, but it's not broken down by month...

Thanks...

I've been dying to post this link for network monitoring systems. Sadly for you, most will be WAY overkill:
http://en.wikipedia.org/wiki/Comparison_of_network_monitoring_systems

I use Zabbix here and have generally been very pleased with it. It seems to have scored favorably on that grid posted above, too. It's WAY easier to set up and maintain than Nagios, too.