The D50 was the entry level camera when it came out. Its not a mid-level (aka: D70, D70s, D80, D90) camera body.
I do agree with Yekcim about the focus motor. I too would look at the next model up just to keep the focus motor in the body. But then again, I upgraded from the D50 to the D300.
If the D7000 is the D90's upgrade, then that would be a nice upgrade. The D3100 wont give you to much more over the D50 other than an improved sensor and a few other small stuff. Not sure if its worth it, IMHO.