One of the keys that I have read is plenty of RAM and a fast RAID 1 or 1+0 rather than RAID5. It seems I/O access is rather heavy from what I have seen on my test system. I have a 4x 7200 SATA in RAID 1+0 that will be moved to 6x 15k SAS in RAID 1+0 for faster access and redundency.
Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now