First the 70-300VR is a lens that works fine on both FX and DX cameras.
I never owned the D300, but I did own a D90 which had similar ISO performance (some thought slightly better) and I also wouldn't go over 800 ISO with it. With the D7000 I replaced my D90 with I would shoot up to 1600 ISO comfortably. I found myself, however, needing to shoot indoors more often in poor lighting often needing to go to 3200-6400 ISO and the D7000 just wasn't giving me the performance I needed.
I can tell you that I would comfortably shoot the D700 at 6400 ISO. Above that you can still get very usable smaller images for web pages, etc. I would print anything I shoot at 6400 though.
Basically my view is that you already have the lenses, and you are used to the control layout and ergonomics of the D700 from using the D300 - this combined with the fact that it will meet your goal of better ISO performance AND it being a very cost effective purchase at this stage makes this a smart purchase in my opinion.