Is your CPU your only bottleneck in the tasks which make you think you need an upgrade?
The Q6600 is very friendly and OCable as far as the 775 chips go. I had two different builds with E8400 E0 stepping CPUs and those are also very friendly to OC. You need to determine first whether you can benefit from a quad core based on what you do with the computer.
Once you upgrade the proc, you will most likely be bottlenecked by other components at that point anyway, so you don't need anything killer. A 6600, 8400, or 8500 should all be fine.
What is it you use the computer for, what exactly are you trying to rectify, and what are the other components (mobo chipset, RAM size/speed/latency, etc...).