As this isn't a Zimbra Desktop issue I'll move it to the correct forum.
The memory usage you're experiencing is perfectly normal in a Linux environment, it will use RAM rather than swap. However, you need to give more dteails about what the exact problem is and details of the load on your server such as the number of user, whether they're Web UI user or IMAP/POP3, what infor does top give you etc. etc. There are also a few recent threads on server load, have a search for them. Has this problem started recently, is there anything significant in the
log files, have you done any operating system updates that may have caused this problem?