It doesn't matter what you set mem voltage (mvddc) to, it's always running at 1.6V.  To actually change mem voltage, you would have to muck around in the bios.  The setting, as you point out, acts as a floor for the core voltage (cvddc), though it's actual purpose escapes me.  I can only imagine that a.) it's a leftover of previous models which is actually no longer necessary, or b.) it's meant to be a sort of signal of the minimum core settings necessary to fully utilize the memory bandwidth available at that given clock setting.

Bottom line, just set it somewhere below your P7 core voltage, and everything will be fine.  In fact, I usually set it somewhere around my P4 core voltage, so I can fiddle w/ P7 while the miner is running and not worry about being limited by the memory voltage setting (which can only be adjusted while the miner is offline.)

Btw, I find OverdriveNTool much more straightforward for adjusting clocks/voltages than any of the branded tools.