I just saw this, so I wanted to weigh in for others looking to do something similar.
On the negative side, the D90 actually has a very slightly better dynamic range than the D700, 12.5EV versus 12.2EV per DXO. That may not be much of an issue in real life I suspect. The most significant hurdle is that the FX upgrade comes with a pricetag (lens costs). The 18-200 is nice though (I'm in the market for one for the D5100 to replace the 18-55/55-200). But FX lenses... that's where the biggest jump in performance lies. The 85 1.4 and the 14-24 2.8 are amazing, it's tough to come close with a DX. Lens costs are roughly DX times 3 or more... and there's the pro-sumer line versus the pro line. Build quality and aperture capability are often the most significant differentiators.
Other than that, the D700 will focus so much more accurately and faster(especially with a great lens), much faster frame rate in RAW, you no longer have to hold ISO to under 800, the DOF will additionally stretch the creative envelope, the viewfinder is so nice, etc. I switched from a D300S to the D700 a year ago (about the same basic thing you have done) and it's a whole new level of capability for me.