I thought the title was Dream Merchants or something similar, but no luck googling. The story was about aliens who arrived on earth with an offer to humans of immortality. Basically, they would implant a microscopic chip of a human's brain, which supposedly contained all the personality of the person, into a supercomputer traveling trhough space forever. The person (chip) was able to pre-program the life story they wanted to live out within the cyberworld including how they would die. They then entered the life programmed and lived it out. This could be repeated throughout eternity ie virtual immortality. The options were limitless and one could choose any time period etc. throughout human history, as well as have other persons appear as recurring characters in the story. Of course, while living out the life all memory of past lives was unknown.