>Consider renting a D4, I upgraded from a d300 6 months ago >and still excited about my purchase today. Its a fabulous >piece of gear. See if it works for you before dropping $6k..<<<
Hands On is the most important thing to me. 5 minutes in the camera store, I knew the D4 was the fastest and most capable camera I ever held. It was also much heavier and more expensive so I came home with the D800. It is only in the lowest light with the fastest action that I question that decision, which is very rarely.
Even a D3 or s would be a huge jump up from a D300, but a D800 might fit your hand better.