For most games 2 cores is sufficient though, for at least the next 1.5-2 years. They have only recently started making use of more then 1 core in games, games using 2 cores are starting to pop up, and as far as I know Crysis will be one of the few games that can use 4 cores. But for more games to follow in Crysis's footsteps, it will take another 1.5-2 years for them to hit the shelfs.
On the other hand, it's not like you're going to be really cpu limited by a 2.4ghz c2d with 2 or even 4 cores. Perhaps you will have 60fps instead of 65, but that's it. And if games do use 4 cores, then 4 cores at a slower clockspeed will surely beat the 2 cores at a faster clockspeed. Besides, if you simply OC a g0 stepping q6600 to 3ghz, you're all set. 3ghz is fairly easy to achieve with a somewhat cheap as7 pro.
That means when overclocking, go q6600, if not, it's a tossup, but if you think you might overclock in the future, it's q6600.