Erik,
By definition, RMS of a pure sine wave is .707 * the peak AC voltage. Since peak is one half peak to peak then the correct method of working back from RMS to P-P is Volts(peak to peak)=1.414 * Volts (RMS) * 2. Remember that RMS is intended to be an approixmation for the "heating" equivalent of an AC wave compared to a DC voltage that would produce the same heat in a load. By using the RMS method, this takes into account the varying AC voltage of a sine wave. Now, some people will say that the average voltage of a sine wave is zero since the positive portion of the cycle is exactly the same magnitude but opposite phase as the negative portion of the cycle. However, anyone can observe that a light bulb does heat to incandescence on an AC voltage. So by observation we know that AC voltage does have some ability to do work.
A quick search gave me this link...
http://www.bcae1.com/voltages.htm. It show the relationship of P-P, Peak and RMS values superimposed over a sine wave.
One thing to note as well, the 120 volt AC house power supply is also measured in RMS. So that gives 165 volts peak and 330 volts P-P. Hence the need to have high voltage insulation in house wiring. As an aside, line voltage operated TV sets use a full wave rectifier and a simple capacitor filter right off the power line, making most TVs primary power supply 165 volts DC.