I also have a D3 and a D2X and I am considerin either 2 options: 1. Keep it. 2. Sell it and get a D300.
In either case, I have a DX body. This means that every lens I own becomes virtually 2 lenses. No matter how you put it, DX remains attractive for telephote shooting. Especially long telelenses like the 400mm get a whole other dimension. 400mm on a D3, 600mm on a D2X! Both at 12MP.
As long as the light conditions are good (becaue of the noise) the D2X is still a great camera. I primarily bought the D3 for its low noise capabilities and the improved AF. If I only shot static objects in good light I would not have bought the D3.