Community Edition=Open Source Edition
I should clarify that, you could set up the Open Source edition to capture mail if your 1st MTA goes down. If you want a true second server install, you should have both running NE, and you'd need to talk to your acct rep for more info on licensing requirements.
whew 256 is cutting it close. You maybe could do it if you only run ldap and mta. . .but that's cutting it real close.