There is an interesting new website called dxomark that has a very comprehensive database of measurements on the performance of different digital cameras. This allow you to do quantitative comparisons between cameras from different manufacturers. Of course, you have to take these things with a grain of salt as invariably you'll find that the most expensive camera is by far the best in these measurements considering noise at high ISO, dynamic range and such. However, if you go out in the field and shoot an image at ISO 100 of a landscape with the camera you would not see any difference to your much cheaper one. And of course, if in such a situation you would shoot using a 4x5 camera with ISO 50 velvia or so, you would get far higher quality images than even the most expensive Canon 1Ds MkIII or Nikon D3. That said, I was curious for a while about what the actual IQ difference is between my D300 and the very yummy D700. Well, here it is. One example is the signal to noise ratio:
which is a full 4 dB better for the D700 at 200 nominal ISO, which corresponds to over a factor of 2 more noise in the D300. This certainly squares with my observation of noise at low ISOs in the D300. If on the DxOmark website, you roll your mouse over the scalebar on the right, you will see what this means for the image. Giving a very similar image to my examples.
Here is the dynamic range:
As you can see, apart from the base ISO, the dynamic range of the D700 is more than a stop better at every setting. That is impressive!
Now if any of this matters to you enough to spend thousands of bucks extra depends on your priorities. I wish I had the financial means to get a D700 and full-frame lenses but right now it is just lusting. I wish it had better resolution though, as from everything I have seen, the D700 gives exactly the same sharpness as a D300. The full-frame advantage doesn't hold up at all in that parameter. For that, use a 5D mkII, or a film camera I guess.