Hi billco,
I second Andrew99's comment. the DX lens is built for small sensors. It'll work on a D700, but reduce the resolution and render the point of an upgrade moot. In any case, the D300 is GREAT, GREAT camera. I'm trying to figure out why you'd want to switch so quickly anyways? Are you unhappy with it?
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At the level of the D700, the assumption is that if your using that kind of camera, you'll have the resources or professional need to invest in the highest quality lenses to go with it.
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Given the quality of the D300, if I were you, I'd consider sticking with the D300 for the time being and investing in some high quality lenses instead. Get yourself set up with a wide-medium (like the 24-70 or 17-55) and a nice telephoto (70-200VR or 80-200 for a budget).
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In the good old days of film, it used to be the case that lenses were much more important than camera bodies, which were just black boxes that let in light. Sharpness was determined by the quality of the glass elements in the lens. For a short time, this equation was inverted. A 3MP camera was MUCH better than a 1 MP camera. And a 6 MP was better than that. And a 12.4 MP camera even better. But as move above that threshold, I think we're returning to situation where your lenses are once again getting more important than your the resolution built in to your camera body.
That's my two cents. Good luck with it, whichever way you go.