That's interesting because the D5100 and the D7000 use the exact same sensor which would indicate the they have the same low light capability. Sensor performance data for different cameras is readily available. As far as I know, DxoMark.com is a reputable site that provides such data. With that being said, I have never used a D5100 or a D7000. I'm just strictly comparing the numbers:
DxOMark - Compare cameras side by side
Look at the last line of the side-by-side comparison. Setting the ISO above the shown setting is supposed to produce visible amount noise in photos.