I know my crappy C3 can saturate 100mbit Ethernet just fine, and can deliver enough data to play back two high bitrate xvid files simultaneously on different clients.
I plan to drill holes in my cheap cabinet to mount an ITX or uATX motherboard. If the C3 can't cut it, I'll probably put in a PentiumM or Turion or Core Solo something or other.
I want to take down two of my four file servers. That's the goal. That will mean building one machine that's capable of dealing with with something like 4TB of data, one way or another. Since I very seldom use the fast CPUs in those servers any more (they're A64/3000s), I'd like to replace them with something that generates a minimum of heat.