Without knowing your SLA requirements and your actual load/day (there's a big difference in resource requirements between getting 1,000 emails a day versus 10,000), the very first thing I would say is to at least run 2x HDDs in RAID1, so that failure of one HDD doesn't take your server down instantly. You might want to increase the RAM, since you can also configure the AV to use a tmpfs (i.e. a RAM disk), and Zimbra does use java so that tends to chew memory.
Other than that, your setup should work fine for a small installation, I have one setup using an old IBM 3650 (Xeon 5420) to run VMware ESXi with an Ubuntu 10.04x64 guest with 4GB of RAM, 2 vCPU and also SATA disks for ~40 people, who predominantly use the webmail interface (I have another setup for > 1000 accounts using multiple ldap, proxy and mailstore servers, with separate AV/AS, but that's another story).
Release 7.2.0_GA_2669.UBUNTU10_64 UBUNTU10_64 FOSS edition