Hi there!
Giant step here, DX to FX,
On FX,
your 50mm and 18-105 are just that,
On the D90, the 18-105 is approximately 27-157mm,
On FX, 18-105 is going to be cropped anyway
And, with the D800 you will need huge storage and larger cards
IMHO,
Plan for the future and buy new glass, lasts almost forever, cameras are like PC's, you need to change them more often
Best bang for your buck at the moment is the D7100,
DX, 24mp, pretty much latest technology,
My friends who are waiting for the forever coming D400 bought the D7100,
they are wildlife photogs and like the "reach" of DX, EG: a 300mm lens is effectively 450mm
if you really need and want to go FX, get the 610 and put the extra into high end glass
Lastly, with all those pixels, your shooting discipline will have to be perfect, poor shooting will produce disappointing results,
Also, with that many pixels, diffraction is now going to be a problem, you will have to choose your f stops wisely
If your still going for the D800,
Get at least one good lens, like the 24-70, f2.8
If budget is stretched, get at least a 24-120 f4, a lot slower glass but will be a good all round lens
Regards,
Gary