Unless you are overclocking the RAM, you generally don't need to know anything about the voltage as the stock voltages are generally fine.
Different RAM technologies (SD, DDR, DDR2, etc) all take different voltages, but that won't mean anything either as they all have different physical slots so that you can't put the wrong one in anyways.
The only time you may care about RAM voltage is if you have some system instability, and you diagnose it as RAM. Bumping the voltage up 0.1-0.2V can cure this depending on the issue.