I wouldn't race out and buy a D3200 to replace your D3100 or any of the other cameras in the D3000 or D5000 series. All of those camera are extremely capable. I had a D3100 for over a year and was very pleased with the results. On the other hand, if someone was thinking of buying a new or first camera, the D3200 is well worth considering as is the D5100. Also, keep in mind that I was using a very expensive lens which should make one consider whether or not money would be better spent on good glass rather than a new body. As for the high ISO range, according to DXO Mark, the D3200 does not have a significant advantage over the D3100. This actually surprised me since I thought it would have tested much higher.
That being said, I purchased this camera specifically for the ability to be able to crop tighter since the D3200 and two lenses will be the only ones that I am taking on a trip later this summer.
Regarding John's comments about an FX body, I think that unless you have a reason for buying an FX camera, that you can do just about anything with this camera. FX is going to require heavier, more expensive lenses and you may not be overly impressed with the results when compared to your D3200. That combined with the initial cost of an FX body is definitely a game changer.