It all depends on what you are gonna do with your computer. My personal opinion is to go with a dual core (conroe's). I would go with the e6750 (the one i have) You can easily OC its 2.66 up to a 3.4 at stable conditions 24/7 (if you have a good case like an antec 900 or coolermaster centurion 5.
So yea. all depends what you are gonna do. If gaming i would say go dual core cause games are still not even to the stages of using dual core yet (except some).
Application overload i would go with the quad core. Like photoshopping, video editing and rendering etc.