Considering the high prices the D700s still fetch (8 or 900 quid), I'd actually consider a D600 instead. Better ISO performance, twice the resolution.. and a warranty!
Downside? It's not a pro camera.. big fat so what
The D600 does make this a tough decision. It offers a lot of performance over the D700 in terms of it's sensor and some nice mod cons, not least of which, video. There are just two things that would personally put me off a D600. Firstly, it is the body itself. This is nothing to do with 'pro' body snobbery, I am along way from being a pro but it is purely about button layout and in particular the lack of a dedicated AF-On button which I find really compromises usage if you like back button focussing. Sounds trivial but it's almost a deal breaker for me as I shoot with my left eye and using anything without an AF-On button off to the right is an pain. Of course for the OP, it may be no issue at all.
The other issue with the D600 is the centrally biased focus points. It just seems crazy that it was released like that, they're near useless for anything off centre. Again, not an issue for many but puts me off.
The pluses for the D600 are numerous though. The sensor really moves things on from the D700 (itself, no slouch). N-photo reviewed every single full frame Nikon camera produced and the current batch offer big gains over the older FF sensors, particularly when it comes to dynamic range, noise and of course resolution.
I suppose it's worth mentioning that the D800 isn't mad money used these days as well...
Anyway, back to the D700/D300 thing. I went from a D300 to a D700 and now (due to having to fork out for a new house) back to a D300. As mentioned, the D700 layout will be immediately familiar, which is a good thing. The view finder is substantially bigger and brighter on the D700. Where it really does excel over the D300 is in low light. I would comfortably shoot at iso 6400 (with a little care) on the D700. Being realistic, I'm getting nervous at iso 1600 on the D300. So probably about 2 stops difference, which in real terms is huge.
I'll not comment too much on IQ, I was highly impressed by the D700 in that department I bought a couple of belting lenses at the same time so hard to be objective. I don't think I had a full frame eureka moment but images are very pleasing. Hard to describe but they seem less harsh, more subtle tones maybe. I notice it most on landscape photos.
If you do go for it, I very much doubt you'll be disappointed.