How can i tell who is visiting my customer's site - and which links they've clicked on

Hi - I have a customer who is wanting to use a service which tracks visitors by reverse IP lookup.
I'm going to inform them that they can track registered user activity, but would like to know what activity they can track - i.e. can they track which user is online in real time and can they track which links have been clicked on by particular users.
Also, is there a plugin for tracking visitors who aren't registered (i.e. the reverse IP approach)
Any help greatly appreciated
Thanks
Mark