Unfortunately you did not wrote what you are doing with your PC. The choice depends on what you'll are doing at your PC.
Let me explain. Most applications does not really well support the multicore technology. Only if the application is designed for multicore technology you'll have a speed increase. Professional Applications like Photoshop or Premiere supports the multicore systems well. Maybe other video authoring systems too.
But finally normal office applications, Browers, eMail clients,... does not really support it. And I thin it is not necessary since this applications are mostly no "number crushers".
For this reason I'd recommend a E8500. You have a CPU with two cores and a higher clock. The Q6600 has a lower clock but two cores more. And if you do not have any applications that support this 4 cores, the remaining cores are ideling and you do not have any benefit with this two cores.
The E8500 CPU is also a newer design and with the 45um technology the power consuption should be lower compared to the E6xxx series. For me the Q6600 is outdated design. If you really need a QuadCore I'd wait a few month until the 45um QuadCores are on the market with fair prices. The new Quadcored also benefit from the new design and have also lower power consumption. The new 45us CPUs are basically not faster than the olfd 65um CPU. The little higher speed comes from the larger cache. The speed will only increase it the applications are compiled with support for SSE4. Currently there should not be so much applications supporting this...
If you'd ask me what to buy I'd get the E8500!