not much to add, excellent responses (at times like this i remember why i would rather be a pentax user, if all else was equal: it seems our kind is smarter :-P )
noise is actually all about signal to noise ratio, this is not a term specific to digital imaging actaully (and, as it ahppens, it occurs at the purely analogue stage of image capture, as a fun fact, or at least most of it does: when the signal is amplified).
because of the linear nature of "digital" image sensors, noise will always come up first in dark areas (less usefull signal), a good explanation of that is found here:
Expose Right
this is only if you are looking to understand more about the nature of noise, predict when it will appear and perhaps avoid it, with any digital camera. for the k20d specifics, i guess it is already clear:
-pixel-to-pixel is not relevant, print or simulate printing at the maximum size relevant to you, to decide what is acceptable and what not (aka: comparing noise from a 6MP sensor to noise from a 14MP sensor, for practical purposes, is irrelevant, it may be rellevant for engineering/measurebating purposes)
-k20d in my experience does better than anything else i have used (d50, k100d, gx10(k10d), some p&s but they don't count), even pixel to pixel (except maybe k100d where they are close), so it does great for practical purposes.
-it is known the extended dr on k20d causes some additional noise in the shadows (though nothing to worry about, from what i have seen)
-if utmost quality and noise control is important to you, shoot raw, disable in-camera-nr, use a good post processing nr method (i occasionally used some wavelet based nr available within ufraw, my raw converter, i was quite amazed at how well it does; others will have more extensive advice on the topic)