Hi, this is my first post here and would like to say that this is a very controversial debate.
Based on all the other Q6600 vs. E8400 threads I've seen, the question really boils down
to what you use your PC for. If you are into benchmarking, video encoding, editing, then I'd
suggest going with the quad core. If you are doing HEAVY multitasking (one guy was running
four instances of World of Warcraft on 4 different accounts), then go with the Q6600.
For general purposes and gaming, I would recommend the E8400.
I personally bought BOTH

a G0 Q6600 and an E8400 and I'm selling my Q6600
as of this moment. Reasons: I am primarily a gamer and use my PC for my classes, I could overclock
to 4.0Ghz on a 90 dollar motherboard, Runs nice and cool, I could run at 3.6 with only 1.2VCore if I wanted.
Both CPUs will be comparable in gaming performance- it'll rely on your GPU a lot more.
One thing I noticed was that the Q6600 seems to handle COD4 a lot smoother, perhaps because
it supports the usage of all four cores. There is virtually no bottleneck from either chip.
The reason why I would pick the E8400 is that by the time you need more than 2 cores for gaming,
there'll already be cheaper and better quad core solutions than the Q6600. As of now, the low
number of games that support quads does not justify the extra heat and power consumption of the Q6600.
If you game and hardly use the 2 extra cores, what's the point? That's my justification for the E8400.
Then again, the E8400 is extremely hard to come by (I got really lucky with buying the LAST one from
a Local PC shop). I got the Q6600 for $210 at MicroCenter, E8400 for $250. Although I paid 40 more for a
dual core, I figure I'll be saving about $10 / month in the electric bill, so in the long run, the cost to me will be less.
Get the E8400 if you can for ~$200. If you can't, Q6600 G0 is still an excellent choice that will
last you a long, long time.