Tue 11-Sep-12 02:21 AM | edited Tue 11-Sep-12 02:39 AM by kentak
As others have pointed out, to get the full advantage of the full size (FX) D800 sensor, you will need to use non-DX lenses. You can use the 18-105 with the D800 in "crop mode," but will gain only a modest increase in resolution over your D90 by doing so.
If you want to upgrade but stay within the DX format, the D7000 is now attractively priced and has some nice feature upgrades over the D90, including 16mp, more focus points, better focus system, better high ISO performance and better dynamic range.
If you want to upgrade to FX without dropping the bucks on a D800, stay tuned for Nikon's announcement this week of the new D600, which is *expected* to be *possibly* similar to the D7000 but with 24mp in a FX format sensor. It *might* be priced as low as $1500, although that might be best-case scenario.
Or, spend the $3000 and get D800, which everyone says is a great camera. Just be sure to budget many extra hundreds or thousands of dollars for the non-DX lenses you'll need. (The same is true if you get the D600).
Or, stick with the D90 and improve your skills and technique a little longer. In the meantime, keep an eye on the new models expected to be released in the coming days and months.
Edited to add: I should have "seconded" the advice about getting additional lenses first. José already mentioned f/1.8 35 and 50mm lenses. The 70-200 is quite pricey, but a good choice if you can afford it. A more affordable choice would be the 70-300 VR, which I enjoy very much. It does not, however, have the f/2.8 capability of the excellent 70-200, but it's also about 1/4 the cost. All of these aforementioned lenses would be usable with any future FX body you might get.