The only real games that were written to be multithreaded are already old enough where it means a difference of 700 FPS vs 640 FPS (exaggerated

). At the moment, the only thing that dual core will do for you in games is offload background windows processes or sound processes (if you're running onboard sound) which results in about a 2-6FPS increase from the single core counterpart, according to all the reviews I've looked at.
However, the Unreal 3 engine is supposed to take advantage of dual core (among many other things), and there are claims that games using this engine will be out by the end of the year. Again, I would doubt that these initial multithreaded games will suffer that much from only having a single core, because they still have to make these games playable on the majority of gamers' systems. Hope that answers your question.