Yesterday I sold my D300 that I had since 2008 and have taken around 17K photos with it. I sold because for my concert photos I need more ISO and D300 stopped in practice on 1600. I always have looked at the D700, as far as I can see it gives not bad at ISO 6400.
I have alas looked into the D3s, but an nice used d3s here in Norway cost the same as an new D800 (around 4000USD/25000NOK), but an nice used D700 usually is around 1650USD (10000NOK)
Im not into video, never missed it on my D300, so that is not an concern for me, only hope my 70-200 VRI isn't too shabby on an FX compared to the VRII.
I won't even attempt to advise you on the "value" of one over the other, but I recently pickedup up a used D3s, adding it to my D700. I shoot a lot of high school sports under outside lights.
In my experience, I've found the images from the D3s at ISO 12,800 to be of noticably (though not dramatically) better quality than those from the D700 at ISO 6400. Mind you, the D700 at 6400 is still very usable and prior to using the D3s, I was very impressed. But even at 12,800 the images from the D3s are not only showing less noise, they also seem to hold truer colors and more natural/realistic contrast.
Only you can answer the value question, but there is a definite improvement with the D3s over D700. Can't really speak to the D800 at all.