http://www.nikonchat.com/nikon-dx-format/

What is the difference between FX and DX format Nikon SLR camera?
I have the right Nikon D 80 and now wants to move all the way to the d3 and they are different formats, I've heard its just a preference, but I'm just not sure what the difference is how I like to know more? Oh, thanks to Steve that is exactly what I was looking =]
* Camera FX has a 36 x 23.9 mm sensor size. As the frame size of the 35mm film. An FX lens is designed to produce a full image circle covering a frame of 35mm or 35mm film size sensor. * Camera DX has a 23.6 x 15.8 mm sensor size. Smaller than FX. A DX lens is designed for Nikon digital cameras use a sensor that APS-C. It is projected image circle is smaller than traditional Nikon lenses and not what is ideal for use in a film of 35 mm or 35 mm, sensor size of Nikon. Unless you need in your super APS-C, is, in my opinion, it is always better to buy a FX lens no matter what you have Nikon DSLR. Please keep this in mind … If you move to the D3/D700 and intend to use the entire sensor must be mounted FX lenses. Check this …. if you use a DX lens on your 35mm camera D3/D700 (frame mode full) will get something like this. http://www.flickr.com/photos/zabong/2899309717/
[No-Edit PMA 09] Nikon’s high-end DLSR ‘D300′ with DX format