Aeolus is a synthesised (i.e. not sampled) pipe organ emulator by Fons Adriaensen. It is a software synthesiser optimised for this job, with potentially hundreds of controls for each stop that enable the organist to "voice" their instrument.
Main features of the default instrument are: three manuals and one pedal, eleven different temperaments, variable tuning, MIDI control of course, stereo, surround or Ambisonics output, and flexible audio controls including a large church reverb.
Aeolus is not very CPU-hungry, and should run without problems on a 1GHz CPU with 256MB RAM.
Aeolus instrument definition file