This is an age old question and I went thru this a few months ago. For me, it seemed better to go with the Quad and Im glad I did. I have gotten into donating my pcs spare time to science thru folding at home and they have a basic program that is for a standard system (single & dual cores) and another program for high end muiti core systems (quads). I have 3 dual core systems and 1 quad. I also have 5 other pcs on my team from different locations that were running for three days before I set up my quad. My quad is steadily scoring points that equals all of the other systems combined based on the workload I am able to turn in. This is partly due to a program limitation for the standard systems. My Quad is allowed to run 100% all of the time and it automatically ramps down on its own when I want to do things such as play games. The standard program runs dual cores around 40-60% and ramps down when you need it to.
For gaming, you can overclock the E8400 a little more and get a little more gaming performance over the Q6600. But I feel this will change when more games are coded to take advantage of more than two cores. This is already happening in some games now and will be more available from this point forward.
You will be happy either way at this point but as I said, for myself I wanted to have more cores for when they can be utilized. I am thinking about upgrading my media center pc to a quad from a dual core to drastically increase my scoring on the folding@home project.
The decision is really yours to make based on do you want higher performance now or later? They will both do what you want now, no problem.
I would have to recommend the quad core due to video editing. Im not sure what program you use but even if it doesnt take advantage of multiple cores, you will still get better performance when you are multitasking with a quad, IMO.