I have the D40x and it's a terrific camera. I chose it over the Canon. To add to the differences above, it also has a higher continous shooting rate of 3 frames per second versus 2.5 for the D40. One key advantage of being 10.1 mega pixel, in my opinion, is that you have greater cropping options whilst retaining picture quality. The better ISO will also give you slightly more picture taking options.
With the D40x you will never wish you had any more pixels or a better ISO and can always shoot with lower quality or higher ISO. With the D40, you might wish you had more pixels or lower ISO. Depends how important £100 is to you really.