Nick, thanks for your comments. I may have been a bit too harsh in stating ISO 1600 is unusable. At ISO 1600 noise is clearly visible in all photos. It doesn't ruin the photos, though it does degrade the quality. There are certainly many cases where I'd prefer a noisy photo over no photo at all.

As far as ISO 800 goes, I've really only used it very rarely. When I was trying to fix my auto-ISO problems (at ISO 1600) I tried all the settings. I remember my test shots had visible noise at ISO 800, so I just never tried it again. Given my limited sampling I can definitely concede that ISO 800 may be quite usable sometimes. Perhaps I'll give it another try.

-Rob