The chaps over at HardwareSecrets have updated their Core 2 Duo E6750 review in order to include some overclocking results:

Core 2 Duo E6750 works internally at 2.66 GHz multiplying its external clock by 8 (333 MHz x 8 = 2.66 GHz). Unfortunately this engineering sample didn't come with its clock multiplier unlocked, so we couldn't increase its clock multiplier in order to have a preview of the performance of a Core 2 Duo with the new 1,333 MHz FSB working at 3 GHz (10 x 333 MHz) - i.e. we couldn't preview the performance of Core 2 Duo E6850.

So we had no other option than increasing the processor external clock rate. During our tests we could put our Core 2 Duo E6750 running externally up to 379 MHz, what made it to run internally at 3.03 GHz, a 14% increase on its internal clock rate. We locked our memories with a 1:1.25 multiplier, what made them to run at 947.50 MHz. With this overclocking the overall performance measured by PCMark05 increased 4.77% while the CPU score provided by this program increased 13.48%. On Quake 4 the performance remained the same.

We could set the external clock above 379 MHz, but the system was unstable. We only consider an overclock to be successful when we are able to run PCMark05 and Quake 4 four times without crashing. Overclocking success depends a lot on the motherboard and on the CPU used. Even between two identical CPUs one can reach a higher overclocking than the other, especially if they are from different production batches. We didn't play with any fancy configuration, like increasing the CPU voltage. Thus with more time and patience you will probably achieve a better overclocking with Core 2 Duo E6750 than we did.