Actually, I was just thinking about this today. I have lots of old freestyle tapes (me rapping and singing when I was drunk) and DJ mixtapes I did. I'm trying to plan it out right now so I can record them all on my PC then burn to CD.
So yeah, the best thing would be to record those cassettes into a program like Cool Edit or Soundforge. Even Cakewalk or Cubase is fine. As long as you can hook up your tape deck to the input of your soundcard then you're set.