If you still have your AF lenses from your D50 then you have lenses designed for body mounted focusing motors (if they were purchased around the same time). The newer lower end Nikons no longer have the body AF motor. A D90 would be an upgrade from your D50 and its a great camera. It is being replaced by the D7000. D90's can be purchased for a good price now. Again its a great camera.
Nikon D type lenses (or older AF lenses) require the body AF motor. Nikon AF-I, and AF-S lenses have a built in motor in the lens, and work in any of the new DSLR's (body AF motor or not).
Nikon D40, D40X, D60, D3000, D3100, D5000 require AF-I or AF-S lenses with motors to AF. They do not have body mounted AF motor drives. They can use the older AF lenses, but they will not AF.