I've read a few different Proxmox (OpenVZ) related threads on here. I set it to 3072/3072 for the installation.
I am splitting up my Zimbra operation to 7 different servers (2 LDAP, 2 mailbox, 2 MTA, and 1 proxy). I don't have a high load on them at all.
My current server has an old AMD Athlon 32 bit on 1 gig of RAM and more than handles all web hosting, voice, email, etc. needs, so I'm not sure I really need 3 gigs of RAM allocated to one of the email servers. Well, until load increases. The "new" box has 4 gigs of RAM.
Further information from the Proxmox forum indicates that Java thinks it has access to all physical system RAM instead of the allocated RAM. The "fix" is to manually set the heap size in Java, but I'm concerned as to how to tell when this needs to be expanded as system load increases.