I'd still buy my Q6600 if I had it to do over again. They do 3.0GHz on stock voltage with the stock cooler with zero problems. I do things other than game though, so quad is more viable for me. If you're getting a chip to keep a while go quad because more games will take advantage of it in the future. The E8400 will likely clock higher when we are talking max, and both will do 3.6GHz with little problems. I think a Q6600 @ 3.0-3.6GHz is plenty of support for any card on the market. On P35 mine will do 3.6GHz by boosting the vcore alone. I don't need to touch another voltage setting on the board.
I think you'd be happy with either and I feel the quad is a better long term choice.
Running OC data collection from users @ XS that includes the Q6600 G-0-
you do deal with a lot of media apps you may want to look @ a Q9450. At the same clock in certain applications Yorkfield quads are 20 to almost 30% faster-
In games @ 1024x it is less than 1% to +7%-
Below is an example of what things will look like when more games use Quad like LP-