JVM configuration, -xmx over 4 gb

I have a request to install a Linux server (preferably Ubuntu 64bit server), and Java (64 bit) on the following machine:

- Intel Core2Quad Q8200 - 2.33mhz
- 8gb DDR2 ram
- 2x 320GB SATA HDD in soft RAID1 mirror (mirror)

The problem is how to configure system and Java because I need JVM to use 4gb or more of memory. This is a configuration and performance question and I am interested in comments if anyone has experience?

What's the problem you're getting?
What -Xmx have you set?

I install 64bit Ubuntu server and 64bit JRE.
When I set xmx over 4gb tomcat will not start. Any Idea?

I tried my car this morning...it didn't start...can you tell me why?

Or put another way, what error do you get in the logs, on the screen, whatever...?

What JVM options have you supplied to it?