, I'm just tired right now and short of patience, so I'm inclined to give short answers... sorry SaMmM
... here's a better answer to your Vcore / overclocking question:
There are no CPU's 100% identical. Every silicon semiconductor nano-circuit is unique, and has slightly different basic properties of electronics such as resistance, capacitance, inductance, impedance, and transconductance, and there are nearly 292 million transistors in a C2D. Although two consecutive serial number CPU's from the same fabrication, with the same stepping codes, may appear identical, they're yielded from different location on the silicon wafer from which they're manufactured, and like diamonds, each has it's own unique flaws.
Even though their dynamic operational characteristics may be very similar, no two CPU's will overclock to exactly the same stable maximum speed, at the same voltage, at the same temperature. Additionally, in a dual core processor, one core will always become unstable before the other. Successful overclocking is achieved through small increments, and requires a methodical approach, and a great deal of time and patience.
Regardless of effort, there are never any gaurantees; we can only say what is typical. Your particular E6750 simply may not be "clean" enough silicon, and may not be cooled well enough to reach your overclocking goal before Vcore pushes temperatures above safe levels on the scale. There's no way to predict an overclock until you try it. That's just the luck of the overclocking draw.