The front side bus is linked with the cpu clock... To get the 2.66 ghz of the E6750, it works like 8(CPU Multiplier) * 1333 (FSB) / 4 (Cycles) = 2.66.
If the north bridge bus speed does not support 333 and only 266 (1066/4) it will not work. Think of it as logarithmic bases? (That analogy makes sense to me)
If you are in the market for a new processor at $200, the E6420 is a good choice and will OC fine. It also has a multiplier of 8, while the E6600 has a multiplier of 9. Hope this makes things clearer...
In terms of OCing, all these processors have locked multipliers, so you have to raise the FSB speed... but to start, they have to both be able to operate at the same base.
Edit - note that it is the the north-bridge on the mobo that determines whether it can support it... Certain mobos which initially say they only support 1066 may in fact be able to support 1333 with a bios update. It all just depends on whether or not the north bridge will accept it. I also found this, last updated August 27,
http://support.intel.com/support/motherboards/desktop/d975xbx2/sb/CS-026614.htm
It does not list the E6750 as a supported processor.