Honestly, while I admire you guys with the talent for scientific analysis, I do not have neither the patience nor sufficient knowledge. So I did this extremely crude experiment, and I admit I did not give it a lot of thought, so please tell me what I did wrong.
I put a cap on the lens, used a manual 1/125s and f/5.6 and shot like that at every ISO. Then, without even loading it into the computer, I checked the file size on the camera's (Canon 40D) LCD and plotted them on a graph. My assumption is that the variations in file sizes have to do with noise only. And, not surprisingly, I got the same results as everybody else: !60 has the least noise, and the best ISOs are 160, 320, 640 and 1250.
Is it safe to conclude that for the best performance (noise and everything else) under sufficient light I should use ISO 160 (and not 100), and then if I need more speed to jump to 320-640-1250?