Jim,
My first attempt at photographing a painting was abysmal. I finally understood that the problem was white balance (WB.) None of the camera's default settings worked to get it right, but the D7 has the WB custom set option, and that worked to set things straight as described in the second post. Minor adjust may be needed, but it wasn't far off.
I just checked
another comparison review I saw and it seems the D5 also has this WB custom set option. Why don't you try that?
Below is a painting I photographed using the WB custom setting with daylight through a window. It is very close to the original in color and contrast.
As to your question about picture quality, it means simply that at a given zoom setting, you won't get as much detail - 2056x1544 pixels versus 2560x1920 pixels. To put it another way, if you wanted the same level of detail as the D7 on a certain portion of the frame, you would have to sacrifice 504 pixels from the length and 376 pixels from the width of the photo.