Being somewhat of a D300 fanatic.....why on earth would you want to get rid of your D300s?? Does it not take good photos for you? Do you not have DX lenses to shoot with? You prefer to spend more money on FX lenses?
Seriously....if I took a picture with a D300 and the same picture with a D700....do you think you could tell the difference? Without access to the EXIF, I seriously doubt it. Main difference? The DX sensor is smaller than the FX sensor.
Having an FX camera does not make you a better photographer...same as having a Ferrari does not make you a better driver!
Making a living shooting with your camera?...sure, go for the FX. Why not. Money's no object, right? You're a hobbyist?...then what's wrong with your D300s? Nothing, right?
Some would disagree with me on the points I just made, but.........I urge you to give it some thought.