To really give you a good answer I would need to know the size of your network. If you have a small network cacti might be your best bet, but as soon as you get to large deployments you have a better shot at justifying a comercial product which will offer more features tehn cacti.

For small networks

Cacti Cube ( http://square.github.com/cube/ ) Graphite ( http://graphite.wikidot.com/ )

For large networks I am going to be biased towards SevOne ( I work there )

SevOne ( http://sevone.com ) SolarWinds ( http://solarwinds.com ) OpenNMS (http://opennms.org )