Don't feel squeamish about pushing up your vCore. Once it becomes apparent that a vCore bump is needed to OC higher, as a rule of thumb, the C2D requires an increase of about .05 volts per 100Mhz.
For example, if your highest stable clock is 345 x 9 (3.1Ghz) @ 1.35 max stock vCore, then you've got about 300Mhz of OC ceiling remaining, so you should reach 356 x 9 (3.2Ghz) at 1.4 vCore. This varies among E6600's. The goal is not to exceed 1.5 vCore and 50c full load on both cores.
The cooling mod link explains in detail how vMch (northbridge chipset) affects FSB stability and OC ceiling, but as a second rule of thumb, it requires an increase of .05 vMch per 10Mhz FSB. For a second example, you shouldn't have to increase stock 1.5 vMch to 1.55 until around 370 ~ 380Mhz. This varies among P5W DH's.
Again, don't feel squeamish about red vMch numbers in BIOS; it's OK to crank it to the max, with the cooling mods installed.
Hope this helps. Enjoy!