My opinion is that when you really "need" an FX camera, you'll know it and not have to ask. For me, FX provides improved ISO capability, wider field of view for wide-angle shots (I already had FX lenses) and with telephoto lenses, it helps with picking up a fast-moving subject (bird in flight.)
As far as waiting for the D7200, I'd wait until it was out for a while before buying one to ensure Nikon's consumer testing goes ok.

After that, I'd get rid of my 7100 for the 7200 to get the improved buffer since the small buffer on the 7100 has frustrated me from the first few weeks that I had it.
As far as choosing between the 8XX and the 750, I'll likely choose the 750 when I upgrade, although the price of used 800s is intriguing, to say the least. Try to determine how the differences in file size will affect you; I know two people locally who had to purchase new computers to handle the files that the D800 produced. Granted, they may have needed to do so earlier, but they really had problems working the d800 files on an older computer.
WM