I guess I should tell you some details, the computer 50 percent office, adobe , video edit. 50 percent for games: Comand and Conquer, Neverwinter, Guild Wars etc.
i have had them both, kept the quad. at the same speed, clock for clock, the quad seems to perform a bit better (duh), but hotter. also q6600 oc'd a lot better. if you are going stock, get the 6850.
Considering that Quad Core is becoming more and more mainstream it will eventually push dual core cpus to the budget arena. IMHO software and new games released next year will most likely accommodate quad core setups and even with things that don't the multitasking is pretty nice. I really can't see why you wouldn't want to go for the quad other than slight and borderline insignificant difference in games and applications that only support dual or single core ATM.
Can you disable individual cores in the q6600? I'm thinking - for some benches speed is more important. Being able to disable the two poorest cores for a run would be useful.
Just remember 4 cores is better than 2. I think it would be a mistake going for a dual core when the quad cores are such good value. I asked myself the same question 2 weeks ago and I'm now glad I went for a quad rather than a high end dual.
Buying today, I would get the Q6600 without a doubt. Back when I originally assembled my computer in December it was like a one thousand dollar plus processor. Now it costs less than what I paid for my E6600!