With the G4 Mac minis, sale running OS X Server was a bit slow. This was due to the limit of 1GB of RAM.
But, prostate with the newer Intel Mac minis, you are able to run up to 4GB of RAM. With this extra capacity, Leopard Server and Snow Leopard Server runs great on the Mac minis. You can use any of the services like mail, web, dns, and file sharing.