It is sometimes better to have 2 JVM running in parallel instead of one...
You should also consider the type of HW architecture you have. Multi CPU, multi cores, ... if you look at the GC algorithmes available since 1.4.1 (talking back to 2003... you might also choose a better one :) ).
Apparently Sun (Oups ... excuse me Oracle) provide a fairly good article http://www.oracle.com/technetwork/java/gc-tuning-5-138395.html
One thing I learned from the people maintaining the systems alive ...
There are very few people that can really help you with a theorical answer.
Just try all the settings that look consistent to you and watch them behave, live !