The mini-Nazgul image is too funny. :D But I think that if someone like Frodo or Bilbo had been possessed by the Ring, he would not have become necessarily a Nazgul or an evil spirit, since they both started with an utter lack of ambition and meanness. Even more so for Frodo if he had been turned into a wraith on Weathertop. He would have become a fleshless and tormented creature, but not an evil one. Tolkien often says that there are many strange creatures around. The Barrow-wights were definitely evil, but now I can't think of disembodied creatures who are not evil in Tolkien's world; anybody else?