Setting memory and GC properties are not really easy and dependent to your application.

Some general:

- Xms == Xmx will free the JVM to recalculate the memory

   will save resources, also the memory is reserved for JBoss ;-)

- MaxPermSize should enough to load your complete App, if the app is complete used direct you will see it after a short time

   the perm area is only used for objects that never dies (mostly)

From my experiance a setting ~2-3GB should enough, sometimes if the application is small and the requests fast you might have a better performance without big (stop the world) pauses with less memory.

Good documentation you will find here:

http://www.oracle.com/technetwork/java/javase/gc-tuning-6-140523.html

http://www.oracle.com/technetwork/java/gc-tuning-5-138395.html