It's much more complicated than we even imagine.The relationship of noise vs ISO depends on many variables but at a given temperature is the same for any sensor and consistent.
Never work with JPG's as the compression introduces noise that is even worse than the real noise.So take home message is start with raw frames, convert to tiff and also do this for dark and flat frames.
Two good resources for info on sensors and noise and comparison to film is
http://clarkvision.com/imagedetail/index.html
and
http://www.photo.net/learn/dark_noise/
I am still trying to get on top of this and I have a degree in Physics! Slowly getting there by learning from others.