Get the D700. You will love it and not regret it for a second. I got one three years ago when it came out and was amazed at the low light/high ISO capabilities. Since then, the D3s has taken low light to the next level, but the D700 still does more than everything I could possibly ask of it. The first FX lens I put on it was a 50mm 1.4g -- and I didn't take it off the camera for almost a year because I was having so much fun shooting without a flash in places I never cold shoot before (dark restaurants, dimly lit parties, concerts, etc) - fast primes on a D700 are like having night vision! You can shoot in ridiculously low light with slow shutter speeds you would never otherwise consider with a D90.
Personally, I am not considering the D800. I don't care one lick about video, I don't need 36MP files for the type of shooting I do, and the D800 seems to be more of an extension of the D3x - high resolution. If the D700 wasn't enough low light for me, I would get a D3s or wait for a D4s/D800s. Short of those three models (two of which don't even exist), the D700 can't be beat.