Mike, Great choice to go the a D800 - you will be so glad you did.
The D800, in DX mode with a DX lens, will unquestionably out perform a D90 with the same lens in every way. If you want to keep using your DX lenses - that's great. Compared to the results from a D90 camera, you can't go wrong with all of your current lenses.
However, understand that that all DX lenses project a smaller image onto the image sensor than an FX lens. This reduced image does even not cover the whole FX sensor. When you put the D800 in DX mode, it will only look at a reduced area of the sensor that matches the DX lens. The data from those pixels that surround the reduced area of the FX sensor are simply not recorded.
In contrast, an FX lens projects the image over the entire sensor, taking advantage of every pixel, giving you the full resolution the D800 has to offer. You can take a full FX frame image, crop out of it any number of DX images, and not loose any resolution of what you would have captured had you originally shot them in DX mode. The cropped DX images will have the appearance of having been zoomed in on part of the larger FX image, but in fact all you did was crop out the extra pixels.
But what is most important is that you get the D800 and enjoy it, whatever mode you want to shoot in!! The D800 produces the best FX and DX images out there. Have fun!!