my middle machine has a joystick/midi port (one of the reasons I chose it) but this is difficult to find now, and not really necessary any more.

cheapest option for midi is a host cable to the serial port, if your keyboard has the required socket.
then a midiman interface or similar to usb (easiest) or parallel port (edirol have nice cable/controllers for usb, though you might find cheaper).
then a pcmcia card just for midi like UD. You can spend more and get a hardware wavetable in the card too for more polyphony or better voices.