Here are a few comments concerning the difference between a D700 and a D300 (and a D200), based on some extensive tests I did. The 300s should perform the same.
1) The resolutions are essentially the same. Any differences are so tiny that it depends on the picture as to which is "better." Even then, it's a strain to decide, and ultimately it's a fools errand.
2) The D700 is the clear noise winner (duh!), even at base ISO. Its high-ISO capabilities are a revelation.
3) The D300 has too much noise in the skies to my liking, even at base ISO (200), and even when you nail the exposure. This is a serious impediment to landscape photography, in my opinion. Manipulate the skies, and the D300 may quickly be hurting. The D200 is much better, but it's base ISO is 100. At 200, it's worse. If you use the D300 at ISO 100, it's better (as good or better than the D200), but some say it clips highlights too easily if you do this.
4) The lower noise of the D700 really does translate to better shadows, and a wonderful ability to pull information out of them.
5) The D700, at ISO 800 and above, is about 1.5 to 1.75 stops better in noise (i.e., you have to be 1.5 stops slower on the D300 to equal it). However, the photos turn out even better than that, since its color fidelity and resolution remains remarkably high up to ISO 3200, while the others start to lose definition and become duller before then.
6) The D700 is far less sensitive to chromatic aberrations, but more lenses will show vignetting. This is a preferable trade-off, in my opinion
Now, an important practical matter:
7) The D700 is significantly larger and heavier (plus more expensive, too). Mate it to the 24-70 f/2.8 and you have a magnificent picture-taking machine with creamy bokeh, etc., but we're talking serious bulk and weight, plus it's got that "he's pointing that huge camera at me" intimidation. A D90 or D300 plus say the 16-85 is smaller and lighter, and a little more discreet. This difference is to be taken quite seriously.
For me, the D700 was well worth it.
I hope this helps.