This is a hard question. Things like SC2, Skyrim, Crysis1 all need huge single-threaded performance because they're not very well optimized for multicore, particularly beyond 2 cores showing much less gain than better optimized titles. So one could say you need the absolute best CPU you can find for high IPC + high clock speed to get a good experience. At the same time, one could also argue that something like BF3 64P MP uses more CPU resources in a more efficient manner (lots more going on to be honest), and that requires the best of all worlds, high IPC + high clock + lots of cores.