64 bit hardware is important because you run out of bits at 4GB on a 32 bit machine. Plug 2^32 in to a handy calculator, and you'll see the crux of the problem.
One of the beauties of systems like Java, as well as all of the abstractions we pile up upon it, is that we can be unaware of the mundane low level details and realities of the systems we're running upon. But, here's an example of where knowing such details can be quite important.
As for what settings are important for adding memory to Glassfish, lookup the -Xmx and -Xmn switches on the Sun JVM (assuming you're running the Sun JVM -- it's potentially different for other JVMs).
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