Hardware and performance

We currently run a small moodle instance, but due to its features and ease of use we are expecting it to grow very rapidly in the near future. 

Are there any builtin methods for monitoring the amount of users, their activities and what the current load of the server is?  If not are there any easy ways to judge if a user is "active" so I can build something to do this for me?

We want to be able to tell ahead of time that things are slowing down so we can make things faster before people notice things being slower...

Thanks,
Mike