Ahead of making decisions regarding hardware, we make thorough
performance and compatibility tests. We've been through dozens of different
boards, CPU's, ram-types, disks, PSU's etc. before launching new hardware configs.
All configs. have been tested as MTA, SQL, WWW, and NFS servers,
both separately, all together and in any imaginable combination.
We have not been able to push any hardware to its absolute limits during normal
to high load or operations. Naturally you can bring the server to its
knees - all hardware has its limit, but it will take a substantial amount
of data-processing, disk I/O and net I/O simultaneously.