Noise

The D5100 has an excellent JPEG noise profile, very clean up to ISO 400 and despite some detail degradation from color noise quite usable up through ISO 1600. Beyond that will depend upon the content of your scene, though I wouldn't recommend ISO 6400 or higher. (Though you might be able to gain some latitude by shooting raw, I wasn't able to test raw processing performance as Adobe Camera Raw doesn't yet support the D5100.)