The D90 is an upgrade over the D50 (actually the D90 follows the path from the D70, D70s and D80).
The D90 has the same sensor as the D300, but in a smaller body. Pretty much the same metering (which is a big improvement over the D50). It has more focus points than the D50 and more frames per second (for those times when your shooting sports).
I came close to getting a D90 a few years back when it was first introduced, but went with the D300 instead. I upgraded from a D50 which I took over 16,000 pics with.
The D90 is an excellent camera, you can't go wrong. It will also use the same lenses your currently using with your D50 and will auto focus with any auto focus lens made for Nikon. The D90 (like the D50) has a focus motor in the body so it will auto focus with AF along with AF-S lenses (AF-S lenses have focus motors in the lens. There are some Nikon camera bodies that don't have auto focus motors in the body,,, ie: D40, D40x, D60, D3000, D5000).