The best comparative shots I have seen are at Imaging Resource and DPReview. The images are taken under identical lighting in RAW and in the case of Imaging Resource, you can download the RAW images to your favorite RAW processing software, disable the noise reduction and sharpening and compare noise levels at every ISO.
BTW,I've done the comparison as I am contemplating a D7100 as a companion camera to my D600.
Here is one of my ISO 6400 shots from a discussion in the D600 forum. The light level in this room was far below the way it appears in this image - I'm sure the D7100 will be good under low light, but it won't be like this.