There is more to it than akxpckwb says.
Think of a ram modules voltage as the power required to maintain or change an electrical state. So 1.5 volt ram needs 1.5 volts of power to change a 1 to a 0, for example. 1.65 volt ram would require that much power to do the same.
RAM is designed to be run at a certain power level. Running at a different power level can damage the ram, or cause the data in the ram cells to become garbled.
Basically, current cpu/chipset designs require that memory operate at 1.5 volts. That's what you really need to know. Don't buy desktop memory that operates at anything other than 1.5 volts.