You may want to re-consider the VM. The hardware you've described is what I'm running (Dell R710 Quad core) with 16GB memory.
I've installed the free version of ESXi 5 for the host and then I have Zimbra on as a guest. It's very comforting to know that, during upgrades, I can snapshot before hand and revert if necessary.
This system is handling around 300 employees.
The BlackBerry users are going to be a pain; to support them, you'll either need to run another server for their calendaring/contacts (BES) or only support IMAP. We strongly discourage the use of blackberry with our user base.
As for archiving, go for MailArchiva, there is a free and a paid for version
Ben Franklin quote:
"Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety."