Nikon iterates its camera models at regular intervals. The "pro" Nikon bodies (D1-D2-D3) have been on almost precisely a 4-year interval, with a mid-generation refresh, like the D1 going to the D1h, and the D2h going to the D2Hs and the D2x going to D2Xs and the D3 going to D3s...
Right now, the D700 could be the benefactor of a mid-life refresh: that is what Nikon has done when it adds a letter to a model name. Jen says she wants HIGH-ISO performance, and the simple fact is that the larger the sensor, the better the camera will be at each higher and higher ISO setting, all things being equal. Well...things are not always equal: Nikon premiered the D3 in 2007, and then in late 2009, Nikon **signifcantly** elevated the D3 model to the D3s model, with better sensor performance at High-ISO levels, a greatly expanded and usable ultra-ISO capability, and Nikon added a few autofocus system tweaks to come up with the D3s, which will be their "speed" camera for another 2 years or so.
Now, this very week, like yesterday, Nikon announced big rebates on a bunch of bodies--that is to help clear the channel of currently in-stock stuff. If one looks at the improved High-ISO performance betweeen the 2007 D3s (which shares the same sensor with the D700) and then looks at the late 2009 D3s refresh of the original D3 model and its peformance--it makes sense: the D700 with an s added could be "the" Nikon light-duty professional body for the next two full years.
OR, there might be a D900 announced in just a few days. Look back at Nikon's camera release spacing: they are on a slower pace than Canon has been...Canon has been releasing their mid-level models every 18 months or slow. Right now is a time to wait for the D700's 2-year refresh,and hope that it gets an updated sensor--the same one in the D3s.
With Nikon, what you want to do is buy a body when it is brand new and state of the art,and the sell it off just when the new model is about to be announced. If you buy a NEW generation model, like the upcoming D4 or D900 or whatever, it will have about a four year life at the top. You can afford to skip a generation, or the mid-life refresh if you want to.