400Mhz is the maximum frequency of the DAC on the video card. Consider for a moment that VGA (that is what you're talking about,) uses an analog signal and the card needs something to make that signal, which the DAC is used for. Not running it at 100% most likely is more to keep the DAC within safe limits within the realm of what it can do which is why you won't see it get all the way to 400mhz output. All in all, as video cards stop supporting VGA I think we will see video cards start ditching the DAC all together which I can't wait for.
Edit: Just noticed that this is an old thread. WHY YOU REVIVE IT?!