We haven't done this yet, but our plan is to put our Zimbra install into a Xen VM on the following hardware:
Just waiting for a replacement CPU to arrive. Then we'll be putting Ubuntu Server 7.10 (64-bit) on there as the Xen dom0, and putting Zimbra into an Ubuntu 6.06 LTS (32-bit) Xen domU.
Tyan Thunder K8SD-Pro (S2882) motherboard
2x AMD Opteron 200-series single core CPUs ~2 GHz
8 GB DDR400 ECC RAM
3Ware Escalade 9550SX-12ML SATA RAID controller
12x 400 GB SATA-II HD
Depending on how the CPU/RAM/HD/NIC usage is, we may be moving our old webmail system into a Xen VM, as well as our old FirstClass server. Keep all our e-mail systems virtualised on a single box (until we finish the user migration to Zimbra, that is).
We've had good experience with Xen 3 in the past (Debian Linux 4.0 dom0) on similar hardware for file/print servers and web server. We're putting together a larger box (Tyan h2000M motherboard, 2x AMD Opteron 2000-series dual-core CPUs) to take over for a bunch more servers.
So far, we've found that you can get away with 2 VMs per CPU, or 1 VM per 1 GHz of CPU (whichever is higher), depending on the task.