If you like playing games then the E8400 will perform better presently, but quads will catch up later when more games can use multiple cores. Encoding is one task that can be quicker on a Quad providing the software in question is programmed to use the extra cores. Quads will also have the advantage when your using multiply programs, making general computing more smooth.
I remember back when Dual cores were hitting the shelf's and they couldn't match the higher clocked cheaper single core CPU's in games, but that has stated to change as many games now use at least two cores. I remember doing a test on my old 4400+X2 and setting the task manager to use just one core rather than two and the game was noticeably choppy. Quads will become the preferred option just like Dual Core did and even if today's game performance is behind the E8400, the Q6600 will still perform well and there's the option of Overclocking the Quad too. My Q6600 hit 3Ghz on a better cooler and a small voltage.
Q6600 is the better overall CPU and excellent value for money.
While I won't dispute the facts this article shows, I find that it doesn't represent real world processor speeds. A Q6600 at 3.6Ghz is 1200Mhz overclock. They need to overclock that E6850 to 4.2Ghz to make the same comparisons.