I have a D200 and a D700 and I can give no opinion on the D300. For a comparison D300/D700 you might want to have a look at www.dxomark.com
This is my subjective assessment of the pros for DX and FX, respectively:
DX: Longer reach due to 1.5 crop factor. Less vignetting. Better results with less than stellar lenses - corner softness is less noticeable.
FX: Better low light capability - D700 about 2.5 stops better than D200. Higher dynamic range - easier to recover highlights. Can use lower apertures before onset of diffraction - D700, f/16 vs. D200, 1/f11. Better colors and better bokeh when using identical lenses.
For me the decisive factor for getting the D700 was the desire to fully utilize the capabilities of the Nikon 14-24/2.8.