All i can say about this subject is.. My Q6600 removed the biggest annoyance I've had on my pc. a slow startup of windows and other programs.
the Q6600 started vista a lot faster than my friends E8400 (his was quicker past the loading screen, but before you can actually start using your pc is what I mean. )
I also run loads of stuff at the same time, hence I'm happy with the Q6600, and wouldnt want to switch it with a dual core. Like DirtyD said, the E8400 could be faster in games etc (altough, only slightly as far as I can see. ) and I say could be, because I recon there is some fanboy-ism involved!
either way, both are good processors, and Imo, both are worth their money.