Its called SpeedStep, once the computer is booted up it lowers the CPU multiplier to save on energy and lower the heat being generated by the CPU. When the extra power is needed (default setting is 5% or more CPU power used under Vista) the multiplier will go up again.
I don't know. SpeedStep usually defaults to an X6 multiplier.
@OP, you can check this watching cpuz as you run something that loads the CPU (an instance of Prime95, for example). You should see the CPU go to full speed.
SpeedStep can be turned off in the BIOS. In the "Advanced Features" menu, look for something called "EIST" (Enhanced Intel SpeedStep) and disable it.