In DX mode, the D700 slightly under-performs the D7000 and D90, according to Bill Claff's tests. Regardless of which nudges out which, if you get a D700 for improved low light work you need to shoot it in DX mode, fill the frame, and use as a fast a lens as you would use shooting DX with the correspondingly shorter focal length.
(that last part about the lens sets up some interesting mathematical comparisons that nullify a lot of FX advantages at certain focal lengths, not all terribly long)
And that would generally apply to any newer FX camera although the D600 will best the D700 a bit.
The magic of the D700 is the sensor size, not the sensor itself. Contrary to some popular belief .
I love my D700 but I would not go around shooting it in DX mdoe all the time. Especially with the choices now and the slightly inherently better D90 sensor.