Ask yourself this question: Am I buying a D700 for bragging rights, just to say I own a D700, or am I buying it for the pictures it can take? Personally, I would get the D300S, since you already have a ready supply of lenses for it. Save the money for newer or better lenses. Seems like a no brainer to me, but hey.....that's just me.
Fotojack, i have never disagreed more than the comment you have given here, which obviously comes from someone who has absolutely no idea.
The d300/300s is a very good camera, no question and will cover most events. It is a DX camera as others have pointed out against he FX format of the D700.
First the image in the viewfinder is larger and therefore very usefull if you are shooting a lot of Macro or low light photography.
High ISO setting will deliver noticable noise at around 1600 on the D300/D300s whilst you can double the setting on the D700 and still get some fine resolution shots. That is very noticable.
These are for me the most noticable and in my opinion important difference between the D300 and D700.
If you are comfortable in selling the DX lenses and replacing them with FX lenses, I would do so and opt for the D700. There will be a time when all cameras will be FX format and the DX format will be phased out. Maybe a good idea to start selling those old DX lenses now, while people are still willing to buy them. Remember Nikon produces very high quality lenses and they do not change the lenses as much as the bodies ( some as much as 10 years).
I would go for the D700 everytime, although the D300 is a good camera too.
PS: I own both