What would be a good step up from a Nikon D40 DSLR?

I'm planning to upgrade my DSLR and I have a Nikon D40 at the moment. The Nikon D40 is alright, but I would like something with more megapixels and higher quality photos, also I would love to have a much better autofocus and light balancing qualities. I would like to take professional quality pictures but I just use my camera for casual personal use so it dosen't have to be too fancy. Which DSLR camera do you think meets my description? It would be more convenient if it was a Nikon.

Well I'm doing a 365 days right now, and mostly using my D40 and not my brand new D7000. I get most shots done.

For non related 365 days I'm using the D7000 of course.

You actually don't need a new camera, just a new lens, like a 50mm af-s or something…

What do you need more MP for? 6 MP is plenty and enough even for smaller poster size prints, and also somewhat forgiving about the lens that you use. You won't really get better quality just by buying a better camera, and more MP are only useful if you also invest into glass that can actually resolve them. E. G. With my 12MP D90 i run into the limitations of the lenses more often than those of the sensor.

What do you mean by light balancing? The exposure compensation of all cameras works the same, all you might get by buying a more expensive camera is that you won't need exposure compensation as often as it might hit the right exposure on its own more often. A wider light meter may be useful… I even have my D90 set to half-stop exposure compensation instead of third stop so that my light meter displays three stops in each direction instead of only two stops (also, i often find 0.3EV to be too little and 0.7EV too much, so 0.5EV is just right for me).

The autofocus really is an issue with the D40 and a good reason to upgrade. You'll find that of the D90 much better (and it also can autofocus with AF lenses, not just AF-S), that of the D7000 to be excellent and that of the D300s to be nothing short of crazy.
If you're a dedicated action shooter, get a D300s. Otherwise the D7000 or, for the same price, D90 plus an additional good lens, will serve you very well.

Nikon D90. If you can push a bit more (budget), used Nikon D300 will keep you happy for a long time.

Obviously you're going to want to be able to use your lenses on this new upgrade, so I'll suggest a few. I wish you would have included your price range…

If you want to go a baby step up, the Nikon D3100 would be great.
But if you want to take a huge step (and more $$$), I would go with the D90 or D7000.