Every CPU is different. For example, I used to have two Q6600's, one did 3.6GHz with 1.35V, the other with 1.45V. The others hitting higher clocks with lower volts have "gems"... and P45's
My memory reaches it's max clock speeds with stock 2.1V, so I doubt increasing that would get you any more headroom. But your RAM should be able to keep up with the FSB fine, so it shouldn't be a problem.
You could try increasing the NB voltage to 1.55V, but I doubt it's the limiting factor. A lot of the 45nm quad cores have "FSB walls", they're not so much walls, more like a thick hedge of thorns once you reach a certain FSB speed, meaning you have to hugely increase the Vcore to get any further.