I have been running the FOSS edition of ZCS since late March of 2010. Since then I have migrated over 400 accounts from another Sendmail server.
This weekend I successfully installed ZCS network over the FOSS version of 6.0.9. Here are my server specs.
RHEL 5.5 64bit
Intel Xeon E5540 @ 2.53GHz
12GB of Ram
6 x 2TB drives in a Linux software RAID1(all 6 drives) for a 100GB / partition. Linux software RAID10 with near 3 for a 3.4TB /opt partition. I can lose two drives from each stripe and /opt will still be fine. 2GB swap file under /.
We are only using about 380GB for the mail store. My largest user has 15GB of email/attachments.