I had a similar situation - wanted a wide angle lens and an update from my D40X - I considered the Sigma, but went witn a Nikkor 10-24 and a D7000 - by getting the lens at the same time as the body, I got some money taken off the price of the lens. I still have the D40X and probably will keep it, too. A friend wanted to buy it, but he really wanted me to give it to him - and I decided that it was worth more than he offered, so I kept it. I'm still using it, too; although I really like the D7000.
If you are going to go up the line, I would advise that you skip a generation or two - for instance, from D40, skip the D90 and go to D7000. From D90, skip the D7000 and go to whatever turns up this summer. There are too many advances to buy every generation, especially when what you have is working and pleasing you.