Is replacing all two prong outlets with GFCI outlets a good idea?

I have an old house that has electrical wiring with just two wires and no ground wire and no ground wire to the metal box. Is it alright to replace old outlets with GFCI outlets. I am also thinking about redoing the drywall in my house and I would have someone replace the wiring and the fuse box (with a breaker box) whenever I do this. Is the rewiring of the house a better way to go? Why are people allowed to sell and buy homes that do not have a grounded electrical system?