Once I got past the sharp 6 MP of the D70 (as opposed to the soft 6 MP of the earlier D100), I had to upgrade most of my lenses to see much difference on the 12 MP D90.
I fooled one picky photo editor who insisted on 12 MP images by simply increasing the "resolution" of a D70 6 MP image in Photoshop. Of course I didn't increase the resolution: I just increased the pixel count by increasing the image size while keeping the same resolution. This was not a model railroad publication, and the editor was young and rather unsophisticated. As long as the image was 4288 x 2848, she was satisfied. Even though the image was actually shot at 3008 x 2000, and was probably published at 1800 x 1200 at 300 dpi or less.
Such is life in dealing with editors.
BTW, I didn't upgrade the lenses. The pro lenses to fully use a 12 or 14 MP sensor are way above my head.