That was the route i took a few weeks ago. I do own a D90 and was looking for a good used D700, and found it (under 8000 clicks). Now on the same boat looking for some cash to get an 70-200 2.8 II. I kept my D90 as a backup/long reach and been loving shooting with both. Definetly, the best investment is on an pro glass.
Nikonians!!! My best investment made after my camera!!!