If you get the D600 then your 18-200 mm lens becomes obsolete and you need to replace that with something else. Also, analyze carefully the type of shooting you do, and will be doing. Full frame cameras are bigger and heavier to carry around, ditto their FX lenses.
I went through this same question when I wanted to upgrade from my D90 three months ago. After a lot of soul-searching I decided that the D7000 was a more appropriate upgrade for me from the D90. So far I have not regretted the decision.