it depends on whether you have any legacy lens or digital lens'. I would agree that the D300 is a great way to go especially if you will stay with digital lenses. If you have no current lenses then another alternative is the Nikon D700 which is full format similar to the older film cameras. The D300 and most other digital cameras have a smaller sensor which makes most lenses longer by a factor of 1.4 or 1.5. So a 50mm lens becomes 70mm, or a 20mm wide angle is really about 28mm to 30mm in 35mm Camera terms.
here is a great resource for camera.
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Jim