How much memory does postgresql require?

How much memory does postgresql itself require?  How is memory allocated?
Is memory allocated when client connections are made to specific databases
and is it released when the connection has ended?   Is ps or top the best
way to monitor memory usage?  Are there any tuning recommendations for
memory?

Specs: RH 7.2, Postgresql  7.2, Interchange 4.8.

Trying to learn Linux, Postgres and IC.

Thanks,
Jeff