When you say 'professional action photography' do you mean sports?
Firstly, as has been said before, if you don't have much experience with photography going professional straight away is expecting a lot of yourself. You'd be best on a steep learning curve for a few years at least.
Secondly, the D50 is a perfectly good camera (I have one myself) and is a great introduction to digital SLR photography, but one of the most important things in action photography is the numbert of frames the camera can take in a second. The d50 only does about 2.5 per second I think, which isn't really that great. But if you want faster you have to pay more.
Most professional sports photographers will use something like a d2h or eos 1-d.