Tim Sweeney from Epic Games was asked what he prefers, a dual-core or a quad-core CPU:
VE - Assuming you were making a new rig to run today and tomorrow's games, which CPU would you recommend at the $266 price point? A Quad-core Q6000 2.4GHz or Dual-Core E6850 3.0GHz?
Tim - If you're buying a CPU primarily for gaming, the sweet spot of the market seems to be dual-core. There, you can get a higher clock rate than is available (at least economically) in quad-core. All UE3 games, for example, scale really well to two cores, and you get significantly more performance than single-core much of the time. Though UE3 and many other engines scale somewhat to 4 cores, the benefits aren't huge.
That will change in a couple of years, but that's another upgrade cycle.
If you plan to do heavy level-building and mod making, I'd recommend quad-core. The development tools scale well to quad-core, having lots of content authoring apps open in Windows benefits from quad-core, and if you're going to be running a dedicated server on your machine to test a mod while also playing on that machine, it helps for that.