I don't know of any pre-packaged rpm or existing how-to for installing MRTG on SME 7.x
You can get similar information from
http://wiki.contribs.org/Sme7admin.
You can get daily internet usage reports from
http://wiki.contribs.org/SargYou can control p2p traffic with
http://wiki.contribs.org/P2p, but there are issues running this package on the latest version of SME.
SME 7.2 is based on CentOS 4.5, so you can theoretically install any CentOS 4.5 rpm with reasonable hope of success... watch out for changes made to configuration files, as SME uses a set of databases to manage configuration files, and rebuilds the files automatically at various times - so you may install a 'virgin' RPM, and it may work fine until you change something in the SME server-manager and the affected config file is rebuilt...