Any Nikon lens that is not designated DX will give you full use of the D700 full-frame sensor. DX lenses will work, you'll lose the benefits of the full-frame sensor (you'll get lower resolution) but you will still have all of the other benefits that the D700 offers - higher ISO, low noise, in-camera editing, etc. I'm in the same boat. If I were to upgrade to full-frame, unless I could make a screaming deal on selling my D80 and 4 DX lenses, I would keep them and use until the budget allowed for adding more FX lenses (I also have two FX lenses).
Mark
