The safe bet with the two Sound categories is to give them to the same film. Sound engineers do the nominating, but it's mostly actors (who don't necessarily know the difference between the two awards) who do the voting, so they usually pick the same film twice. For the record, "Sound Editing" involves just the effects, while "Sound Mixing" awards the total blend of music, effects, and dialogue. This year, there are four films that overlap in these categories, so we can eliminate Iron Man from Sound Editing and Benjamin Button from Sound Mixing. That leaves The Dark Knight, Slumdog Millionaire, WALL-E, and Wanted.
The other thing to keep in mind here is that while the Academy loves to honor prestige films, this is the place where big loud blockbusters get rewarded. If they have the opportunity to award a big loud blockbuster with prestige and critical acclaim behind it, all the better. That's how The Bourne Ultimatum and The Matrix won their Oscars. This year, the film that fits that bill is The Dark Knight - the highest grossing movie of the year, critically acclaimed, thought-provoking, good acting, and lots of noisy action.
As for Visual Effects, voters usually look for a film where the effects aren't there for their own sake, but are integrated into the storytelling in a meaningful way. In this case, I think that Benjamin Button's more subtle, character-transforming use of computer elements will trump the flashiness of Iron Man and The Dark Knight, which had more traditional action-film effects.
In the Film Editing category, voters have the option of awarding the stolid workmanship and good storytelling of dramas, (Benjamin Button, Frost/Nixon, and Milk) or flashier high-energy films (Slumdog and The Dark Knight). Guess which ones usually win? While it's possible that The Dark Knight could approach a sweep in the technical categories (like The Matrix did in '99) or that Benjamin Button could pick one up on its road to a sweep, I'm going to pick Slumdog Millionaire. It straddles the best of both worlds: fast-paced action and suspense that expertly cuts between time periods while including dramatic moments with breathtaking transitions. And it has dance numbers. Shiny, happy dance numbers.