There is absolutely no problem with iso 3200 expansion. As far as I know, you also have to turn iso expansion on 1-D series cameras (which will expand iso to 50 and 3200), but somebody correct me if I'm wrong. Today, I shot few images at iso 3200 in low light, but overally it was sunny day (the dog I shot was in shade and I didn't want to use the flash, and I got decent shutter speed at iso 3200) and when I loaded the images to computer, I just asked myself "where did the noise go?" However, if you'll shoot in the dark, then noise will be more obvious, but still you don't have to worry about it much.
I'm afraid I cannot understand your second question.