Custom user levels

Hi all

I am trying to set up some user privilages. I want 3 levels for front end users, top level can view all topics, middle can view most topics and lowest level can only view a few topics. All the topics are displayed in a list of links, users should only be able to view links to topics they are allowed to access. Does anyone have some pointers or resources on how I would go about doing this?

Thanks
Luke