I have used the 24-70 and the 70-200 extensively with both DX and FX camera bodies. The lenses perform beautifully on both types of camera. Switching from your D90 to an FX camera will have a much smaller effect on IQ than switching from your 18-200 to the f/2.8 zoom lenses without changing your current D90. In fact, the IQ might be slightly better with a DX camera, since only the center area of the lens is utilized.
The main benefit of an FX camera is the better high iso performance, you get decent captures even at iso 3200. Also, due to the higher MP you will be able to crop more aggressively.
Most importantly, your lenses will have a different "reach" on FX: At 24mm in FX you will get the same angle of view as at about 35mm for DX. This might be beneficial for your landscape photography. On the other hand, you will have less reach at the long end.