With modern integrated circuits, especially those in a PC, voltage levels are typically about 0-1.85V, so you're off quite a bit there. Now, voltage with no current means no power consumed, think about a capacitor. If you charge it up and then disconnect the terminals, it will remain charged to that voltage, but there's no current (open circuit) so no power is consumed.
CMOS (based on MOSFETS) works similar to this. The gate of a MOSFET works on capacitance (technically electric field, hence the field effect in the name), and ideal CMOS circuits only use power when they are switched from one state to another (vs TTL which each gate will use power when in 0, but will not use power when in 1).
Memory is always active in standard PC systems. Sure, having more modules will draw more current. Memory modules typically will take a max of 1000mA when doing heavy operations, and at they typical 1.85V that most of these modules are, that's about two watts of burst power, but nominal draw is much lower. Overall two memory modules in a system is a small percentage of the overall TDP of a notebook system.