^James' point is very valid. To elaborate, 400MHz is the default 'goto' speed for most video cards when running in 2D (desktop) mode. They slow down to this speed purely to save energy, which is an option that can be disabled if desired. In ATI cards this can be achieved in Catalyst, but being unfamiliar with nVidia cards for some years now, I couldn't tell you if the same holds true for the nVidia stock software.
RivaTuner is another option but again, I'm not certain if this program allows such a tweak to be applied. Really, all this unfamiliarity and conjecture doesn't make my post very helpful I suppose.