You don't neccesarily need to increase the voltage to the memory modules in order to overclock them. I have my PC2100 memory modules running at almost PC2700 speed at 'stock' voltage.
Just increase FSB and see how things go - when you hit a wall in terms of o/c, memory voltage MAY need to be increased then in order to bring stability to the memory modules, which in turn may allow you to o/c your setup further. Could be however that your cpu/mem has simply hit its max o/c. It all depends.