I am a long time user of Nikon film cameras and am looking to get a used digital camera. I am new to digital photography, and see that Nikon has several very distinct lines -- major lines like D2, D200, D50, and so many products in each line. Can someone help me understand the major characteristics of these lines? Once I understand the broad differences, I'd be able to study the details of the individual models. My needs are not professional, but I am serious about my art. Regards.