Now, back to what I did not realize: There is no way to add a second network adapter to a Mac mini. Apple does not offer one, nor do any third parties. I called tech support to inquire about my options and decided that running Leopard Server with only one network card was not a very good idea. I abandoned the project.
Five minutes of Google:
Mac USB-Ethernet Adapters:
I understand Apple's USB-to-Ethernet adaptor should work on other Macs running Leopard 10.5.2 or later (Daniel Sumorok has verified this)
Will PC World pay me?