The 400mm is a huge lens, especially with the 1.5x crop the D300 gives you. I've used the 400mm, but only on a mono-pod, as it's way too big to handhold for more than a few minutes at a time. And the DOF, wide open, is razor blade thin.
I'm not sure what you're using the 400mm for, but it's really built for shooting sports like football, soccer, and baseball, where you're in a fixed position. The 500mm f/4 seems like a more popular lens for wildlife shooters, as it's a lot less weight.
David Buzzard