Well- The quad Cores are too new right now to give much of a single app speed increase. Unless the app supports splitting it'self between the 4 cores that is.
If you do lots at the same time- then the quad is the way to go. Waiting for a while will proberly get you better.
If you want to play 1 game and maybe music or somthing else- then the Dual core is better.
Overclocking is a extra factor.
For example- overclock the quad to 4.2GHz
That means that you have a total power of around 16.8GHz!
But the dual can only pull up around the 10GHz mark.
Conclusion: (yes- i have made a conclusion
If you overclock- get the quad for sure.
If you don't- get the duo (faster gaming) or the quad (better multitasking- and better New games)
Also- take cooling into consideration.
If you take the quad and overclock- you'll need a fairly good cooling (water, or a HUGE fan) and a few other fans to get the air out of the case.