Good fisheye lens for my Nikon D40?

I'm looking for a good fisheye lens under $500? I found this one,
http://opteka.com/opt65.aspx
Is that one good?
And will it work with my Nikon D40x?
If you have any recommendations? The lower the price the better.

As long as you understand that that Opteka is NOT a high quality lens, you will not be too disappointed.
The two best ones are the Nikkor 10.5 mm and Tokina 10-17 mm zoom. Of course with higher quality, the cost goes up

The lenses ace mentioned are good. This one is not at all bad either and it's only $270: http://www.amazon.com/...002LTWDSK/