Topher Grace left the show for Spiderman 3 because he was an avid comic book reader and huge Spiderman fan as a kid, and he jumped at the chance to play a character from the franchise. Fortunately, he was able to schedule some time away from filming the movie, to film a few last scenes for That '70s Finale. For this reason, Topher does not appear for the "Final Goodbye" - scheduling conflicts. It is worth noting, that both Topher and Ashton wanted to leave the show after the sixth season but decided to stay on for the seventh season, which was supposed to be the last season. However, due to the show's stellar ratings, Fox decided to make an eighth season, without these two leads.
Hope that helped.