in most cases the lens is much bigger of a factor than the camera as far as image quality goes...not counting resolution (there is a noticeable jump from 6mp to 10mp). so you may want to consider that...most people start with a standard/cheapish cameras, build up a nice lens array, then buy a nicer body and use their old body as backup (usually pros do this). but to answer your question yes, they have the exact same mounts.
Same lenses, no problem. If you're starting out - get the D50 as it's more than good enough for the time being and you'll have more cash for tripods, lenses, memory cards etc. Then you can buy the D3000 or whatever when it comes out!!