ISO performance are an important thing that differ quite much today. I have taken some test photos with the 5D mark 2. Some in normal ambient light and some dark with the Body Cap on.

Here are a picture from the pictures taken with the body cap on. The exposure vas 20 sek and the in camera noise reduction was turned of. Then I crop out a 31*813 px bar of the whole photo. Then I pushed the photos so the noise would be more apparent.

I would say up to ISO-400 there are no noise at all... Then between ISO-500 - 2000 there are not much, maybe even up to 3200 but then it gets a bit noisier even if it's fairly good up to ISO-5000. Then why the exposure are much warmer at ISO-5000 is an mystery. I guess I have to shoot some more test shoots at ISO-5000. The size of the files also show how the noise are growing with the ISO. The diagram show that the file size start to grow exponentially around ISO-6400.