Nikon's professional camera that is high quality and great resolution (full frame, 24.5 MP) is the D3X, but it costs $8000 without a lens.
The D700 is an advanced camera (also full frame, 12.1 MP) and is $3000 (body only).
Then the D300 has a smaller APS-C sensor (12.3 MP)and costs $1800 (body only) and is the camera I would recommend you strive for, unless cost is not an object, which should have you looking at the D3X.
Then there's the D90 at $975 (12.3 MP)(body only), and the D90 is the next step up from your D40 on the basis of a wanting a more professional camera.
So it depends how much you want to spend. It's the same with lenses but you need more than 1 of those for the range of image types you want to be able make. Fashion and portraits would use the same lenses but you really need at least 2 for those kinds of work and they need to be high quality and fast.
Making quality macro images also requires quality glass but a macro lens can be used for fahion and portrait too.
Your current lens will work well for landscapes as long as you use it in the middle apertures, say f/8 to f/16.
Hope that helps.