I'm sorry everyone has been ignoring your purpose. For sports /fish shooting, you should go D300s.
Not because of your DX lenses, the reach, or higher fps.
FX exchanges ISO noise for lower DOF. If you're shooting fish at f2.8 on your D80, to get the same DOF you need to be at f4.2 on a D700. I assume jumping fish don't hang in the air for you to slowly AF.
FX doesn't magically give you cleaner high ISO. FX and DX are different. FX isn't automatically the way to progress. Otherwise, why not go on to MF, which is only 1.5x bigger than FX? It's like comparing a bike and motorbike by only looking at max speed.
Setting equivalents