#1. "RE: D700 upgrade, can I use slower glass?" In response to Reply # 0
Galapagos is one thing, but for normal wildlife shooting, DX still offers a resolution advantage at this point in the technology and price/performance cycle. I.e we can't always get closer to wildlife nor is it practical to add a longer and longer lens at the really long end of things (there is a large weight, cost, and handling drawback to getting the same reach on FX).
For wildlife we need a 27 megapixel FX sensor with a 12mm DX crop in order to have the same resolution and even then we would need 115 focusing points to equate to the D300-sized focussing point. For controlled wildlife situations where you can get close to wildlife (like the Galapagos or bird blinds or feeding) yes the high iso FX is an advantage, but for uncontrolled wildlife photography you may miss the DX "reach".
SteveK My Nikonians gallery 'A camera is an instrument that teaches people how to see without a camera.' -- Dorothea Lange
#2. "DX +300mm 2.8 vs. D700 + 80-400mm f/4.5-5.6" In response to Reply # 0
Monteverde Cloud Forest, CR
The D700 can probably compensate the 2 stop light loss @ the long end of the 80-400mm but not the lost of IQ, focusing speed & accuracy, downgrade from VR II to VR, darker viewfinder & lost of extra reach TC's can nicely yield on the 300mm.