Rules of Thumb for Autovaccum

What rules of thumb exist for:
* How often a table needs to be vacuumed?
* How often a table needs to be analyzed?
* How to tune Autovacuum?

I have a large DB server, and I'm concerned that it's not being
autovaccumed and autoanalyzed frequently enough. But I have no idea
what proper values should be?

A related question: If I INSERT a large number of records per day,
similar in nature to the existing records, does that require new
vacuum? new analyze? Or do I only need those for DELETEs or changes to
the nature of records?

Finally: What type of performance impact can I expect from vacuum and
analyze, in general?