Absolutely skip the 3100 or 5100 and get the D7000, it is the best thing Nikon has ever made, and the sensor is excellent for night shots, but for normal photography work, its metering and flash control just cannot be beat at the moment.
Not an overly comprehensive review, but a good read is here:
http://www.kenrockwell.com/nikon/d7000.htm
Ken is a fruitcake, but most of the points he makes about the D7000 are spot on.
I wouldn't trade my D7000 for a 5D MarkII or a D3, and I use both at work. The D7000 is an absolute blinder of a camera and is worth the extra few hundred, and as stated, you can use nearly any Nikkon/Nikkor lens made in the last half century, which means you can grab some lovely fast manual lenses for astro work.