It really depends, if you want balls to the wall gaming, go for the E8400! If you want decent (saying that, the Q6600 won't have a problem with any game out now!) gaming, and the ability to multi task better and / or use resource hungry programs, go with the Q6600.
I asked myself this a few weeks back, I went for the Quad. In fact, it wasn't really much of a decision. I went for the Q6600 for many reasons;
You can OC it to around 3.4Ghz on air, which is more than plenty!
I do a fair bit of graphic design and 3D Rendering, so it benefits me more there.
When more games start to use multi-threading, I'll be set.
It's really down to what you want from your computer.
Hope this helps.