Portrait and auto ISO to 1600 test on D5100

It's the same "grain" you get by turning up the ISO. The only way to improve that is to improve the SNR which means you add more light.  Diffuse light coming in a window is great.

For a portrait, especially one at a higher ISO, you shouldn't sharpen the skin at all and some selective NR would be a good idea too. Areas higher in detail don't show noise as much but it will look horrible in smooth tonal gradients like skin.  I don't see much of a noise problem with this image, it would probably print fine at 11x14.  I did a B&W at that size recently, i ADDED grain to it and you still could barely see it.

Go shoot some 400 film and come back and tell me how bad the noise is in this image