Full frame has the same dimesions as 35mm camera format.
these might help, so you can see how the d700 and d300 do not have the same sensor
The only other thing that I miss with the dx sensor is that extra reach, which is especially useful with wildlife. My 300mm lens on the d700 stays at 300mm, but as most people will already know, on the d300 it becomes a 420mm
Its all a matter of whats best for you as the photographer, I much prefer the full frame now, but hey ho thats my preference....
Its not a case of buying the lastest camera and getting the best results..