I wouldn't consider going with the FX format if you plan to use only DX lenses. The D7000 would be better suited for that.
However, if you are looking for a new body that will give better IQ that your older camera AND you plan to upgrade your lenses over time it might make sense.
As to the other question posed, whether there would be an advantage to putting the camera in DX mode with FX lenses to get a bit more reach . . . I would argue against it.
If you have "too much stuff" in your frame, you can always crop in post before printing. But if you have "too little stuff" you can't add it later. In other words, if your lens was too wide when you took the shot, you can crop. If it was too narrow when you took the shot you can't reverse-crop, there is simply nothing there to add in later.
I would suggest shooting it in full FX, which allows you to crop it later in post when you have the time to sit down in front of your monitor and get it just right. Of course, YMMV.