http://www.anandtech.com/cpuchipsets/intel/showdoc.aspx?i=3038&p=8
If you're strictly building a gaming box, you'll get more performance out of the dual-core E6850. However, if you do any encoding or 3D rendering at all, the quad-core Q6600 is a better buy. Our pick is the Q6600 and if you want to make up the performance difference you can always overclock to E6850 speeds, but the chip only makes sense if you're running apps that can take advantage of four cores. As the chart above illustrates, those applications are almost exclusively limited to video encoding and 3D rendering.
http://www.xbitlabs.com/articles/cpu/display/core2quad-q6600.html
So, we think Core 2 Quad Q6600 seems to be a much more attractive offering these days than Core 2 Duo E6850.
However, those computer users who do not want to mess with processor overclocking may think differently. In this case Core 2 Duo E6850 with 25% higher clock speed performs better in a lot of applications, including games. Moreover, it is also more economical. As a result, the most optimal choice for a system working at nominal speeds would depends a lot on the type of tasks it is intended for.
Nevertheless, we shouldn’t forget that most upcoming applications and games are being designed with multi-threading in mind. That is why Core 2 Quad Q6600 may be a more promising solution even if used in its nominal mode.