To be honest, the Quad is more powerful. However, most games that require immensely fast calculations (such as Crysis) tend to run ever so slightly better on higher clocked dual core processors (because their processes are spread over 2 cores instead of 4), such as the E8500. On the Quad Q6600, they still run absolutely fine, and the difference between the two processors is indestinguishable in everyday use. Unless you get out that microscope to look at it, you won't really notice a difference.
For everyday usage, the Q6600 is a better bet. Because most people will use it for more than just games, it is more suited. If you're an avid gamer, you'd probably get something like a QX9850 instead. Its about Quad Core @ 3.2Ghz.
To be honest, unless your an avid gamer, you really won't notice a difference. If you are an avid gamer, you'll save for a QX9850, and you won't have posted this question in the first place.
I have run Crysis on the same computer, just with two different processors (just so happens they were the two in question here). I couldn't notice any difference between the two, except that the Quad was better when recording with Fraps, because there was more processing power to spare.
Hope this clears things up for you two.
