I recently upgraded my D50 for a D300S. I've used the D50 for about 3.5 years or so (Christmas gift when it came out). It's a great camera as you probably know.
It's night and day between the cameras. You'll still be able to use your SD cards, which is nice, because that's one thing your don't need to buy right away. Also, if you have DX lenses, you'll be good to go (as opposed to jumping to a FF camera).
I didn't realize how much more I had to learn with the new camera (but in a good way). The settings and controls that this camera gives you compared to the D50 is amazing. And the first time you shoot the 7 fps you'll be floored!
I don't see a need to upgrade this camera for a long time now (which makes the wife happy!)