Assuming you are using a 64-bit OS and a 64-bit JVM, you can try the latest version of Java 6u13, as I believe that the client-side support for 64-bit applications has been improved.
Note. I would not suggest using more than 1/2/2/3 of your memory for the Java heap. You may also need more main memory to see a real improvement in the amount of memory that you can actually use. (Since you need to leave memory for the OS, other programs, and your own native memory / Java libraries, etc.)
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