I have a D700 and it is by far the best SLR I've ever owned and in my opinion the best and most well rounded high end (sans built in vertical grip) dSLR ever made. However, I don't see how a $2500 camera body will serve you in taking better shots at soccer games. If anything, the wider FoV of the full frame sensor will be a negative, as all of your lenses will have less reach.
Again, I second everyone's attempt to promote the 70-300 VR. When switching to Nikon from Canon, I looked closely at the 70-200 2.8 VR and decided to pick up a 70-300 VR and an 85 1.4 AF-D instead. I seldom shoot events or in conditions that necessitate a 2.8 telezoom, and while it's an exceptional lens for fast action and has very nice bokeh for portraiture, the 70-300 VR is a better choice as a general telezoom walkabout (and actually provides better corner to corner IQ for landscapes on full frame sensors.) For action in daylight, the 70-300 holds its own to the much heavier 70-200. At the end of the day, you really can't tell the difference in the images between the two (with the 70-300 stopped down a bit.)
If it's arena soccer, then yes, you'll want to take a look at a 2.8 telezoom -- but keep in mind that the Tamron or AF-D models will focus considerably more slowly than an AF-S (or Sigma HSM) model. However, if you're shooting outdoors, the 70-300 will likely serve your needs very well at 1/3 the price (and at 1/3 the size and weight.)