I have both the older 50 f1.8D and the newer AFS 50 f1.8G. The lenses are equal on a still shot - I can't tell them apart in viewing actual photos. Maybe slightly different color, but neither is preferable to the other. However, the newer AFS lens is much faster and more accurate to focus when shooting action. I haven't done scientific testing on it, but in practical use of the lens, on a D7000 for indoor basketball, I get a significantly higher keeper percentage with the AFS version. My AFS 35 f1.8 DX is also much better focusing than the older 50 f1.8D. I'm not sure it's that the AFS motor is faster than the screw drive, but perhaps it's that the newer AFS lenses are just better optimized for the newer digital bodies.
I debated on getting the AFS 50 f1.4, when I bought the f1.8 last year. I'm glad I didn't. The f1.4 lens is reputed to be somewhat slow focusing (and for my use of this lens, focus speed is paramount). I think that if you really think about it, the practical applications of f1.4 over f1.8 are very specific and rare. My advice would be to get your D600 and use your current 50mm lens for a while. If you are dissatisfied in some way, then consider upgrading. My guess is that you'll probably prefer to spend that money adding something new to your kit, instead.