We've had two permgenspace problems in the last two days on pear. A bit of research turned up a stack overflow answer that suggests the use of -XX:+CMSClassUnloadingEnabled in the java options.

I also found this (old) page which outlines things quite succinctly. To sum up:
-Xms and -Xmx should be set to the same number because "making these settings the same value prevents the JVM from frequently re-sizing the heap as it grows, something that can trigger frequent garbage collections and can degrade performance."
Only adjust permgensize if your other settings still produce permgenspace errors.

Another, more recent page goes in to more detail. It too argues that "it is recommended that the initial and maximum values for heap size be set to the same value. This is often referred to as a fully committed heap and this will instruct the JVM to create a heap that is initially at its maximum size and prevent several full garbage collections from occurring as the heap expands to its maximum size."

When we get the old-db-server-cum-new-tomcat-server ready we'll try this setup out:
Oracle 1.6 JVM, latest Tomcat (right now that's 7.0.25) and use the recommendations from tomcatexpert.com. We then plan to stress test the new setup with jMeter (jmeter.apache.org).