MOSFET stands for Metal Oxide/Semiconductor
Field-Effect Transistor. (This is to distinguish it
from the JFET or Junction Field-Effect Transistor.)

Anyways, the role of the Metal Oxide is to
insulate the gate pin from the rest of the body
of the device, which is generally silicon,
but it can also be Gallium Arsenide (GaAs -
these are often referred to as GaAsFETs).

The insulating oxide layer is the reason for
the really high resistance between the gate
pin and the source pin. It is also the reason
there is a highish capacitance between the
gate pin and the source pin.

A field-effect transistor works by setting up
a conducting channel from the source to
the drain when a voltage is applied to the
gate. The voltage at the gate (which is
insulated by the oxide layer from the body
of the device) sets up an electric field within
the body of the device. When enough voltage
is applied the channel is complete and
current can flow.

The value needed to activate this channel
is called the threshold voltage and it varies
from configuration to configuration. About
4 volts is a reasonable value to assume.

A really nice primer on using MOSFETs is
given in Nelson Pass's article on building
the A75 amplifier. You can download the
PDFs of part I and part II of the article here:http://www.passdiy.com/legacy.htm.

Another way to calc the highest needed voltage.
VOLTrms=SQR(WATT x OHM)
This value is the RMS voltage. A sort of average for the whole sinus wave.
To get the peak voltage, where Sinus wave is at its maximum.
You just multiply with 1.4