I disagree the clockspeed is still the thing to look at, how many cores and feats of the CPU's are just more variables to take in before deciding what to get... the clock speed still is a very good indication on how fast a CPU can process information.....
I dunno, the Core 2s seem to have far better performance than the old P4s, even though the P4's clock speeds were pretty high.
It may depend on what applications you use. It's still the case that high end graphics and games are more likely to use the cores to their full potential than other applications.
When you have applications that are designed to use all of the cores, then you'll definitely see the difference.
Even without specially designed applications, though, having 2 or 4 cores makes sense, since the OS can more easily handle applications that hog the CPU.
With my four cores, I'm defintely albe to do more. I'm listening to music while playing a game while running a bunch of other applications with no problems.
I also have more freedom when it comes to CPU intensive apps - If I want to say, do some 3D rendering (I play around with raytracing occasionally), I can easily stick the rendering on one core and not see any slowdowns at all while using other applications. I could never do that and expect reasonable performance on a single core system.
So even without the applications that are designed specifically for it, having at least two or four cores makes an enormous impact on my ability to run whatever applications I want whenever I want, even if they're CPU heavy applications.
. . . and if you have applications designed to take full advantage of the cores - the number of cores essentially becomes a multiplier: Rendering using the raytracer definitely is approximately 4x as fast when I allow it to use all four cores. Minus some overhead, of course, but it's faster than I'd ever be able to accomplish on a single core system.
