Each processor has a maximum clock speed that it will run stably at. Lets say that your E6600 can do 3.33GHz stably, or 370x9. Now, there is nothing wrong with this except that you aren't getting the maximum memory bandwidth. Drop the multi down to 8 and now you can do 416x8. The final speed is the same yet the second clock gives you more memory bandwidth for better performance.
The option in the BIOS will be CPU Multiplier, located under Advanced Chipset Features or a similarly named menu.
Does memory bandwidth = memory performance? Why doesn't everyone with a C2D lower their multiplier to 8? How do I know if this is best for me, and are there any other BIOS changes to make when adjusting the multiplier?