Im no expert by far but I just purchased a brand new D700 as my first foray into DSLR. My previous digital was really a very upscale point and shoot but I wanted more control and I wanted to use my Nikon lenses. I had a couple of FM2's and always wished there would be a digital that would give me the ease of operation of the FM line.
Im still struggling with all the menu selections which should be a piece of cake for you. I struggled with the idea of the D800. I may go there some day but for now I think the D700 will fulfill all my needs. Its fast and actually very forgiving. After reading the Nikonians Mastering the D700 Ive discovered a lot of great things this camera does and the latitude you get with the various Picture controls. Its like having a whole pocket full of various films that you can change out for every picture if you want.
If you want the bigger size original file which can only add to your photographs. I understand you can go quite large with these files and there is a lot of depth in DR. You may even be able to use some of your existing lenses.
Just my thinking on the subject I hope it helps to hear why another person went with the D700.