kurt:
We all have heard that a non CPU or a old film lens will make a 1.5 increase image size on a digital camera, so I did a little test. All shots were taken with a Nikon D300 at the same distance and at the same subject, although it did fall over once. First one with the camera set for non cup 400 mm F3.5 that is the lens I have on it a Nikon 400mm IF ED f3.5 film lens.
Here is what Nikon have done.
You say you are using a Nikon D300 that has the DX Format sensor which is 1.5 ratio to 35 mm with that being said a 300mm lens will be equal to 300 x 1.5= 450mm lens most canons are 1.6 to 1 making the same 300mm lens a 480mm then the Olympus is 2 to 1 which makes the 300mm a 600mm lens.
Some of the camera makers such as Nikon and canon are producing a full 35mm frame sensor now.
Don't be fooled by the 1.5 to 1 it is known as a crop factor and that is what it is it is cropping the image down from a 35mm frame.
As far as the quality of the image is concerned it all revolves around the quality of the glass you are using. No mater how expensive your camera body is you will only get image results equal to the glass you buy. If you buy a cheap lens then you will get images that reflect that fact soft focus bad contrast etc.
My New Avatar is in memory of Nancy my darling wife of 10 years who passed away on Monday November the 16th 2009 after an illness
My photo gallery http://thekiwi.org/photography/index.php