And one other note (which has been touched on by a couple of threads in the IDS forum)

If you need to run a sniffer yourself for IDS purposes (or indeed any other purposes such as billing in the case of ISPs, traffic monitoring etc) then you need to either use a managed switch which has a capability of using a "monitor" port or something where it sends all the traffic, or install a hub.

The easiest way is to stick a small hub with only a few ports between your main switch and your router or firewall, then stick the IDS/ sniffer box off there.