Chiming in here to add my tuppence.....
Yes, it's really important to determine what kind of shooting one does the most, as that definitely should guide one in one direction or another.
Last August, when Nikon announced the two new cameras, D3 and D300, I read the specs on each and mulled over the possibilities. I finally wound up buying both cameras because I had specific uses for each. Almost a year later and having used the two cameras extensively, I can say that for me I made the right (albeit expensive!) choice. My usage is: D300 for all birding and anything that requires long lenses. The D3 for candids, portraits, macros, close-ups, wide-angle shots and everything in between. Admittedly, I already had the 300mm f/2.8 VR at the time I was buying the two new bodies, and I had already determined that I very much enjoyed birding and wildlife photography.
If I were faced with the decision today, and didn't already have a D3 and didn't already have a 300mm f/2.8, I would weigh the pros and cons of each choice, but probably would end up buying the D700, as I suspect that this in the long run will provide more bang for the buck. If it is anywhere near as good as the D3 (and I'm betting it will be!) this could be an excellent tool for further exploring photography and expanding your creativity in a way which buying the 300mm f/2.8 VR just wouldn't be. As someone else has mentioned, how often would you be using the 300mm f/2.8? Daily? Weekly? Monthly? It definitely might be the wiser choice to simply rent that lens when needed.
Good luck in making the decision!