Just finished reading this and I see three things from the original post worth considering
Firstly, the fact that you have a couple of DX lenses and don't want to upgrade your glass, well if the D700 crops images with DX glass, you are paying for a D700 when you won't always be using all your pixels. So in this regard, it seems the D300 would be more suitable.
Secondly, you mention you are interested in high ISO. But how high?
There is a big difference between ISO 1000 and ISO6400, yet both can be considered high, particularly when comparing it to the current D200. {edit - fixed typo, should say ISO 1000}
Lastly and most importantly, you currently shoot with a D200.
So have I been but I am now looking for a D300 / D300s (because of the better battery usage) / maybe D700 if I can afford.
So what is the difference in ISO performance between your current D200 and the D300? From all accounts, it is huge, yet this point hasn't been mentioned in this topic yet.
So the difference between the D200 and the D300 versus the D200 and the D700 might not be THAT great in comparison, I am not sure.
It all comes back to my second point - how high you want to push the ISO while still getting great results.
Having read all this, it seems to me the D300 would me more suitable for your needs depending on whether you are looking for REALLY high ISO...
In my situation, my only DX glass is the Nikon 12-24
So, does anyone know, Is it possible to force a D700 to full frame with the 12-24 DX lens and only use it from say 18-24?
And on that point, perhaps you can use your DX glass on the D700 and accept not being able to use the full zoom range? I don't know about this and perhaps someone can advise.
Hope my insight is useful.
And for all you people with a D700, &^%(^& !!