I went from the D70 to the D300 to the D700, so fwiw here's my thoughts...
D700 ups = no 1.5X crop factor, so no having to convert focal lengths if you come from a 35mm film background and you can get wide angle shots without too much distortion (depending on lens choice), a bit easier to throw background out of focus with any given lens compared to crop body (thinner DOF), it also has great image quality and noise performance but most other current models are fine in that regard too.
D700 downs = bigger, heavier, lose the crop factor advantage at the longer end of your focal length.
Honestly, I was a bit of a wide angle nut for a while and had a hard time finding a really wide lens that I liked for DX, but my infatuation with wide has since subsided a bit. I'd say if wide angle and/or portrait shooting is your thing and you don't mind the added heft to schlep around, go for it. Nice camera. But if you're more of a long shooter, have any amount of DX glass that you're attached to, prefer to have a camera the shoots video, or prefer to travel lighter, keep what you have and invest in glass. The D90 is a fine camera. I actually kinda miss my D300. I do believe sometimes there's a sort of romantic notion urging us to go full frame, but with the great features and specs on today's DX bodies, and a decent selection of ultra wide glass, I personally wouldn't feel like I'm missing out on anything.