Tomcat 5.5 - Java and Maximum memory

[danr19]

Tomcat service doesn't start with memory parameters beyond 1536 MB.

The error given in the log is the following:

SEVERE: Servlet.service() for servlet invoker threw exception
javax.servlet.ServletException: java.lang.OutOfMemoryError: Java heap space

It's a Tomcat 5.5.12 with JRE 1.5.07 running in a Windows Server 2003 STD with 3 GB RAM and Xeon 3.2 GHZ.

Does anybody know how to fix this problem?

Thanks in advanced,

danr19