#3. "RE: noise characteristics at 800 iso for the D7100" In response to In response to 0
Al, since you have the D7100, why not take some pictures at ISO 800 and evaluate them yourself? None of us know what subjects you expect to shoot nor what you will be doing with the resulting images. For example, if you will be taking photos of your kid's daytime soccer game and making 5x7 prints, you are unlikely to encounter noticeable noise in the results. On the other hand, if you are taking sunrise landscape images with a wide dynamic range, pulling up the shadows in post-processing, then making wall-size prints, you may find some shadow noise that you consider objectionable. All in all, a few test shots would tell you more than anything else.
Frankly, I think people are way too hung up on noise, obsessing over amounts of noise that will be essentially invisible in the final output.