Another thought if you are considering biting the bullet for what currently is arguably the best DX camera in Nikon's lineup, DX7000; there is the highly rumored D400/D7200/D7300. In my opinion, that should be announced no later than spring of 2013. Then you would have the opportunity of a better DX camera or less expensive new or slightly used D7000. I jumped for the D7000 because I needed it for a particular event.
In my opinion, I think that I would be very unhappy shooting with a DX lens on a D700. In my opinion, if you go D700, you should be thinking FX glass. Can 5.1 wonderful MP be better than your current D90 12.2MP?