I'm seeking to install Zimbra (OSS or Licensed version, any suggestion is welcome) in a server with the following specifications:

- Processors: Two Intel Pentium 4 CPU 2.80GHz, 1024 kB cache

- RAM: 1GB

- OS: Red Hat Enterprise Linux ES release 4 (Nahant)

- Disk set: 68G, 76M, 505M, 244M

I would like to know what is the active/concurrent user accounts i can handle with a server like this.
I expect to manage a lot of user accounts (approx 4000), if the above cannot handle such quantity of users, what should i add to my system?

Thanks in advance.