Maximum allowed memory for Tomcat 6.0 and Tomcat 7.0

Currently I am using Tomcat 5.5 but it has a memory limit of 2 GB max. As my application requires more memory, I am thinking of upgrading my tomcat installation.
Can any one tell me the maximum allowed memory for Tomcat 6.0 and Tomcat 7.0? Is there any other benefits of using Tomcat 6.0 or Tomcat 7.0 over Tomcat 5.5?
And what the effect in 64 bit system?

I don't think that's a Tomcat limitation; it sounds like you're using a 32-bit JVM.

Welcome to the Ranch!

A 32-bit JVM will only allow up to 1.5GB. This is a limitation for all 32-bit applications. It doesn't matter if the operating system is 64-bit or not.
If you use a 64-bit JVM (you will also need a 64-bit operating system), you will not have this limit. As a result, Tomcat can use a lot more memory. At my work, we use 3GB for Tomcat at the moment.