"The 70-300 is f4/5.6 IS. Very interesting replies. I am using a Sigma 18-200 non IS lens and though I try as much as I possibly can it is just all but impossible for me to get a steady image though I am doing what is advised and use f8 on Aperture Priority selection"
Tom
You are probably getting camera shake, because if you use aperture priority the camera is choosing the shutter speed (which is probably too low), with moving subjects you need to control the shutter speed. Therefore if you select shutter priority and a speed of about 500th sec or higher you should get sharp images as you are literally freezing the scene. This will work fine in normal lighting conditions but when the light fails you will run out of aperture to give a correct exposure. When this happens you will have to make the camera more sensitive by increasing the ISO rating (the higher the number the more sensitive) This will allow correct exposures to continue. As with everything there is a limit to what the camera can do, if you increase the ISO too high the pictures will start to become grainy.
The VR lens you describe has two f stop settings, the 5.6 will be at 300mm and f4 at 70mm. f4 is twice as fast as 5.6 (or a full stop in photographic terms) So when using 300mm the lens lets 1/2 the light in so it may need the VR in low light conditions to get a sharp image.
Another thing to consider is that telephoto lenses used at maximum aperture will have a very narrow depth of field, or in other terms only a small area in focus, making background and foreground blurred.
I am not trying to put you off this lens, it is a very capable modern lens, all I am trying to say is that like everything in life you get what you pay for.
The very best lenses like you see sports photographers using are f2.8 with massive front elements they are the absolute dogs b**locks when it comes to quality but they cost in excess of £5000 !!!!!!!!
Hope this is of use.