I have been using a D700 for over four years and I have no desire to "upgrade" to a newer model which will be "old" in a couple of years anyway. If you can get a good D700 for $1,200, I believe that this is an excellent opportunity. I suggest you sell the 18-200 DX lens and acquire a nice FX zoom. The FX equivalent for the 18-200 is the 28-300. According to your gallery, you like to photograph birds and you might also want to consider the 70-300. I do not recommend using a DX lens on the D700.