Hi,
why do you want to set the memory to a high value?
You may use 32 MB for linux and 128 MB for the native heap of the JVM so you could set Xmx to nearly 900 MB if you want to and if you have no other applications running.
Take a look on Heap Settings in JVM Settings and Debugging and enable the GC log for some days or weeks, it will show you the memory usage. Or check the Java Memory on a regular basis on http://yourserver:9090/index.jsp .
LG