Speaking as a q6600 owner I can say that unless you're running truly multithreaded software, don't waste your money on a quad. At this point in time, very few titles will actually use all 4 cores. The only app I regularly use that actually exploits the full power of the chip is x264.exe Other may say "be future proofed" which I don't agree with either. In 2 years, I'm sure you will be able to buy a quad or octa chip for the same price that will out-preform the current generation of quads. That said, if you do a lot of video editing and x264 encoding, you'll really enjoy a q6600.
Just my 2 cents.