The Q9400 is now only $100 more than an E8400 and I wonder if it might be worth it for future proofing and multitasking. I don't need the bonus cores yet, but since later processors would probably require a new system anyway, could I spend $100 now to avoid that?
I expect the E8400 is better when not taking advantage of the extra cores, but by how much? How much hotter is the Q9400 than the E8400, famous for its overclocking and stability?
What do you usually do on your computer? If you only use it primarily for gaming, I would say E8400 is enough. But again, if you want future proof, you might want to wait a bit for the Nehalem (which LGA775 is being discontinued).
Personally I favor quad cores over dual cores, but it really depends on your applications.
Will a Quad show significant improvements in multitasking? No, as most games ans applications don't use more than one core.
I will say that the Q6600 is an outstanding value if you do start to use programs that use more than one core. It's a great OC'er and worth the money. With a good motherboard the Q6600 will OC by .5 gig or more.
Nothing in computers is future proof.