The JVM won't detect 64-bitness, but you can (and should) install a 64-bit JVM. If you go to the JDK 6 download page, you'll see that three of the versions you can grab are AMD64 JVMs for Solaris, Linux, and Windows. Such a JVM will know all about 8-byte pointers and other such goodness.
Since I'm not lucky enough to have the use of such a beast meself, I'm not 100% sure of this, but I believe that telling Java (Tomcat) how much heap to use just uses the normal Java command-line switches (-XmxNNNNm, etc) ; their acceptable range of arguments is just bigger. [ November 13, 2007: Message edited by: Ernest Friedman-Hill ]
Originally posted by Pat Farrell: Are there switches I need to use to tell Tomcat that it can use 5GB of Ram?
Depending on your OS and (if Windows) the way you've installed and are running Tomcat the method for changing your heap setting differ. How are you starting Tomcat? [ November 13, 2007: Message edited by: Ben Souther ]