I want a new telephoto lens for moving fairly fast moving objects such as boats, tennis players etc

I have currently a sigma 70-300 f4-5.6 macro, not a fantastic lens as many of you will know
My camera is the plain and simple D40, but it is lovely

I have under 200 to spend, so my budget is not big
I have looked at the nikon 55-200mm f4-5.6 VR lens for around 130

Also I have looked at the nikon 50mm f1.8 at 100, less on ebay

Any other suggestions, and what do you think of these two lenses