Haj,
For me the decision not to go FF when I upgraded from the D80 came down to not only future lens choices but what I already owned. In your case, you've got a 18-200 VRII which is the lens that I use (albeit the VRI version) 99% of the time. Would you get better results shooting with a prime? I don't know. I know that I tried shooting with primes and hated it, but I'm lazy and thankfully completely comfortable with my laziness. But generally speaking everything for the FF bodies tends to be more $$. At least that's what I found when I looked into it and I'm just not willing to rebuy the kit that I already have. Maybe someday, but not anytime soon. FWIW I also asked a couple of pros and their advice was to stick with DX if for no other reason that it yields a lighter setup.