[silvestrov]

The Mac Mini is restrained by I/O, not CPU, when providing mail, print and file serving.

I think the missing piece is a Mac Mini with a Light Peak connection for external drives. http://en.wikipedia.org/wiki/Light_Peak

A Mac Mini with 1-2-3 external drives connected using Light Peak will be more than sufficient for most small companies, and this will be much cheaper than an Xserve with the same amount of redundant disk space.