The D50 is the D70 only the D70 has more buttons..
They're exactly the same..
Don't go for the D40, please no!
I use a D50 myself and the camera is perfect for everything! Its the lenses where you want to spend the money.
Then again if you more new camera and your budget allows it a D80 is a good option, but it has more functions then the D50 and the question is will you use them?
Anyway I would spend less money on the body and more on the lenses, thats what I did..
Go Nikon, join the Darkside!