I think you need to put it in context. The D700 was released in July 2008 so the technology is 4 years old. But it's a very good camera - trumped only by a few newer or much more expensive models. The D700 is probably capable of doing 95% or more of what you can do with a D800 or D4.
But to me, the question is what's next? Do you intend to keep using the D700 until the successor to the D800 is released around 2015? Are you looking for a D400 to compliment a D700? Are you looking to add mirrorless capability?
A DSLR is a computer with a lens attached. 4 year old technology works pretty well, but will it still be okay when it is 6 years old? If not, you might want to bite the bullet sooner rather than upgrade late in the cycle.
I would not upgrade unless there is something to be gained now or in the near future. If you make large prints it's easy. If you do a lot of high detail photography I would probably upgrade. If you need improved color and dynamic range you might upgrade. I would not upgrade for low light performance although the D800 is a little better. And you might want a frame rate and crop factor of a D400 to compliment your D700. But I would think about your preferred kit longer term and decide where you want to be.