If very high ISO is not that important for what you shoot, personally 99.9% of what I shoot is at ISO 800 or less so I am very happy with my D300, then consider this:
D300 + 17-55 = D700 + 24-70 - $2,000
Okay, not really true, you get a little more FOV range with the D300/17-55 combo.
It is that longer end focal lenghts that bigger, heavier, much more expensive glass is needed for FX. Those 600mm lenses are not airline friendly. I love the DX reach/advantage so I plan on being a DX only user for a long time
Albert J Valentino Nikonian Moderator Emeritus Vantage Point Images Mastery of Composition is the Key to Great Photography