Rory, As Jeremy says Netsight polling interval is a bit long. We use Cacti which is also open source and poll about 10,000 interfaces once every 5 minutes and keep 90 days of granular data. Jeremy also mentioned netflow. You did not mention what type of switches you have and if you have XOS switches then you must use sflow and not netflow. Sflow is built into the hardware so it does not affect your switch performance and there are open source collectors that you can use that will show you at IP level where your internet bandwidth is being used. I have used one before
http://www.inmon.com/products/sFlowTrend.php and it is free.... You are limited to number of devices and how long the data is retained... Two years ago we invested in Solarwinds and sorry it is not cheap but for us having a mixed bag of switches and routers and vendors it has become an important tool we use daily.