The major difference between the two cameras is the sensor - the D3000 uses an old CCD, once used in the D200, D80, D40x, D60, and about a milion of entry level Sony models. Not really up to modern standards when it comes to low light performance. The D3100 on the other hand has a nice CMOS sensor, which up to ISO 1600 is just as good as any other APS-C sensor on the market. It has also the video mode of you're interested. But if the change is worth $200 to you, that's really something you have to decide.
A quick advice though. If you do not own any Nikon lens, you could also take a look at the entry level Canons. The Rebel XS should cost about the same as the D3000, and what you get is a CMOS sensor, not much worse than the D3100's, limited to ISO 1600 though. The Rebel XS also focuses with any modern Canon/Sigma/Tamron lens, has more controls (a dedicated ISO button, no idea how Nikon can make a DSLR without one), Live view if you're interested, and a few other things the D3000 lacks - exposure bracketing, mirror lock-up etc. Not saying that the D3000 is a bad camera, but you get more for your money with a Rebel XS. That's why I have one.
And if you're thinking about the D3100, be sure to take a look at the Rebel T3. The difference isn't as big here, but again - the autofocus capabilities, exposure bracketing, external controls make the T3 worth considering.