FWIW, NASCAR Racing 4 and 2002 were designed as multi-threaded apps and can take advantage of SMP. Does it make a difference in gameplay? Hard to say.
IMO, SMP capable gaming offers some interesting opportunities for game designers who are already pushing the limits of current (and likely future) video cards. Eye candy is cool, but stronger AI, campaign, and physics engines might make some games more interesting for longer. Most of the games I see today are just graphically enhanced retreads of the games we were playing 5 years ago.
terry