Rothi,
I would suggest not to use too big heap size, its very difficult to debug the jvm, if anything goes wrong, I see your capacity is more then enough, I would suggest split the box into 4 instance using additional IPs, and use software+ hardware base clustering, you can also use apache as a front end to do a mesh cluster.
This way your single instance footprint will be smaller and manageable.
you can try various JVM settings which is best for your env, some of the examples are there in my white paper, you can also try some of the suggestions given by Peter J, they are valuable inputs which is fine tune based on years of pensive work.
Post the problems which you face, we will be happy enough to help you out.
Best,
Ruchir Choudhry