Well, one user has just migrated 650k users to Zimbra so scaling is not a problem
With respect to whether you would need to upgrade your hardware is dependant on the activity profile of your users. Do they just use IMAP/POP3 ? Do they use the standard HTML client or Ajax ? Ideally you should be monitoring your server anyway so that you can build some trends of CPU/Memory usage. With respect to licensing you are currently using the Community Edition so are you planning to switch to the Network Edition version ?