If you want a wide angle, get the Tokina 11-16mm f/2.8. If you are on a budget get a Tokina 12-24mm f/4. Both come in old versions (cheaper) or new versions (with autofocus motor). If you get the old version you will have to focus manually on your D40 (not a problem with wide angle).
If you want a fish eye, your best option is the Nikon 10.5mm f/2.8 fish eye. It is expensive, but it is worth it every bit. It is very sharp and will let you take photos in low light. A cheaper option is the Samyang 8mm f/3.5 fish eye. In the USA it is also marketed as Bower, Opteka, Rokinon, etc.. because of a trademark dispute with Samsung. The Samyang is a good fish eye, but very soft wide open. I find it only good in bright conditions or in combination with a flash. It is cheap though.