Just drop any linux distro (even Ubuntu) on an old crusty machine. There are many docs and howtos on the net for setting up just about any game as a dedicated server on linux - as most every game has a linux binary for it (even for win only clients).
You will get far more greater bang for the buck on old hardware with linux as a dedicated server for anything - from ftp server to a game host. If you used Xubuntu for example, you would still have a minimal GUI and plenty of resources left over for running all kinds of dedicated server applications.
What's the old computer specs? I thought the one you listed above was the new one you are putting together. If I misunderstood, then what you listed could easily run Ubuntu (or Kubuntu), it's 100% free, there's a wealth of docs out there, and you would join the many hordes of server admins already using linux for everything.