WHY do you need a full frame(FX) camera?
I have been taking photos for about 25 years now starting with Canon manual equipment. When Canon forced me to completely redo my camera system for AF I just moved to Nikon. The old FD lenses would not work on the new AF cameras unlike Nikon. Since I had to buy everything over moving to AF I went with Nikon.
The Nikon glass I bought and still use today was bought in the 90's with one exception which is a 50mm f1.4 lens I bought last year to take to WDW.

I would guess 3-4 years back I finally moved to digital when I bought a D200. At that point I figured the DSLR's had matured to a point that the next model camera would not be that much better. The D200 felt like my old F100 as well.
All of my "old" FF aka film lenses worked just fine on the D200. And in some ways maybe better since the DX sensor is using the sweet center spot of the lens. Of course the lenses were now 1.5 times longer which is good and bad. Its good at the long end but not so much on the short end so I guess it just works out.
I was happy with my D200.....
Can you see guess what is next?
I ALWAYS have needed faster film/ISO speed. I tried to take college level gymnastics photos with Porta 800 film pushed to 3200.

Spent over $300 on film and processing for not one good frame.



But I learned alot.
I still had a need for faster ISO and when Nikon announced the D700/D3's I about had a cow.

I did not see faster ISO as being a camera feature that would improve so much. I figured it would be mega pixals of which the D200 was fine for me. But oh that higher ISO.....
I bought a D700 and sold my D200. I am regretting selling the D200 BTW.
The D700 has taken photos that the D200 or my old film cameras just could not have done. Its the faster ISO that made the difference not the FX feature on the camera.
So back to my first question. WHY do you need a FX camera? Or do you need the higher ISO that is in the Nikon FF cameras?
My two cents would be to buy good quality FX lenses that you can use now on your camera. If you should ever need a FX camera they will be better and cheaper tomorrow compared to today. DSLRs are just computers to which we attach our lenses.
The lenses I have that I bought in the 90's are worth as much now as when I bought them. The only zoom I have is the 35-70 F/2.8 which is a very good lens. I was shooting basketball over the weekend with this lens and the D700 which was a perfect combination. The D700 had the ISO which was needed. The 1600/3200 ISO photos are very good. And the FF made a difference since if the camera was a DX that 35-70 would not have been wide enough.
The 24-70 F2.8 lens is $1,800. I paid $650 for my 35-70 F2.8. Focus speed was not an issue at all. Worked just fine. No way I would sell the 35-70 f2.8 to buy the 24-70 F2.8 for three times the money.

Even with the D200 my "old" lenses focused as fast as needed for soccer and other fast moving subjects.
So I would say invest the money in the lenses if you can then worry about the DSLR. Nikon sure is making the fast glass very expensive over the last few years so that maybe easier said than done.
Later,
Dan