Coding Horror has a great article on Choosing Dual or Quad Core. Nicely done because it's simple, basically a list of screenshots of the CPU meter playing various games and doing various apps, and letting you know how much improvement you get if you go from a dual core to a quad core (something I've been thinking about).
Hint: not worth it unless all you do is run either rendering software (POV-Ray) or benchmark apps. The list of apps isn't huge either, at least, it didn't include my Battlefield 2142 crack. Interesting to see that about half the the apps tested actually lost performance from 2-4 cores.