Hi,
Sun's JDK contains am small Java-App called 'jconsole'.
You can run it remotely after configuring your tomcat like this:
http://tomcat.apache.org/tomcat-5.5-doc/monitoring.html (Enabling JMX Remote)
Consider reading the JDK-docs also.
Real memory usage can be understood much better with this
As I'm sure you know this is something of a large can of worms...
All I can say is that from my experience of this modern JVM's are getting much
better at working well with the default gc options (ie jdk 5.0 and above).
We use jrockit for our systems, and just use a simple:
-server -Xms256m -X
