Unless there are dramatic optimisations in the Saturn emulators, I think it would be a cheaper alternative to get another Saturn rather than a powerful enough PC. Does a fast enough PC exist that would run any variety of NiGHTS at decent speed?
Fabien, the author of Satourne, got 12 to 14 fps with an Athlon XP1700. I think that was prior to full emulation of the SCSP or the 68000. I have no idea with SSF or Giri Giri project. I read french much better than japanese.