Manfred has followed the road I will also take when it comes to upgrading. Reading several reviews and after having had a discussion with a camera salesman at ORMS, over the weekend, it seems to be the best option to invest in the best glass available rather than investing in a body and using “inferior” lenses. Slapping a Nikkor 24-70mm (FX) lens on a D3200 will prove to be a beter investment rather than having a Sigma lens fitted to a Nikon D800 body.
From what I can gather the D800 requires you to use only the best of lenses available to justify investing in a D800. It seems that perfect sharp focus on the D800 is the critical factor in getting most out of the cameras magnificent resolution.
Benny, my advice to you would be to rather invest in very good FX lenses(going FF in future) and use it on the D3200 body, unless you have the funds to invest in a D800 body and some lenses.