For FX system (camera and lens makes the system) to become affordable to an amateur you will have to wait for a couple of years. And I doubt it will ever really replace DX, just like 6 by 6 never put 35mm out of market.
18-200 lens is the most convenient glass on the market. There is nothing, not even close to it, in FX range (try to imagine 27-300 FX lens, and how much would it weight if it existed). Overall image quality of this lens is very good, and usability is perfect.
On the other hand if you are going for D90 it would really be a shame not to try 18-105 kit lens. It has image quality no worse than 18-200 (18-105 is a little bit sharper if you look really close), and build quality is only slightly behind (the significant difference is that 18-105 does not have metal mount, and that really does not matter if you do not switch the lens more then 10 times a day), and it costs 5 times less. And, for complete set, you could get Nikon AF-S 70-300 VR for the money you saved on 18-200, or, if you prefer wider angle, currently you could get Sigma 10-20mm f/4-5.6 for a very good price.
Getting your gear is never a simple choice, but most of us are enjoying this, but what really matters is taking pictures, that is all that stuff is really all about.