I am sure many people will tell you to "go for it" - this is the D700 forum after all However, I also went from a D300 to a second-hand D700 last year, and have a note of caution. Yes, the D700 is a cracking camera, and the pictures from it are great. Not so great, perhaps, as the D800 with its extra pixels, but the D700 is just as good now as when it was released, and everyone thought it was fantastic then.
The "however" is (from my point of view) that it is noticeably heavier / bulkier than the D300, especially once you start to upgrade the DX to FX lenses and you want longer lenses to get that extra reach you get used to with DX. Also, unless you want the shallow DOF from shooting wide open on FX, I am also not sure that you will get much overall increase in IQ compared to the D300 and 17-55.
You didn't say why you are thinking to make the change to FX, and what you require from the system. I suggest you are clear about priorities, and be honest about things like limits you want on weight / costs etc.
I am actually thinking about going back to DX, because I find I prefer to carry a lighter kit.
Also when thinking about your decision, bear in mind the mythical D400 might be released later this year