[Dovecot] FreeBSD tuning for a dovecot + postfix server ?

Hello

I've googled around to tune a bit my mailhub ( AMD64 FreeBSD 8.1, 12 Gb RAM, 2 Tb raid5 disks , ~4000 mailboxes unix users ) but I am a bit confused,

All my clients use thunderbird as MUA ( IMAP, IMAPS ) to connect to the mailhub no direct access to the machine.

Any of you guys has some pointer to give ? thanks a lot