While there are the real numbers that everyone likes to base enlargement on - - - more important to me is the content and expectations for display of the print. While I have printed many prints up to 20x30 using files from my D70 (a 6MP camera just like the D40 which is a camera that I also now own), a while back I printed 2 much larger prints for clients and both they and I were very pleased with the results. I think that I would not hesitate to print these files even larger if I had a need to.
24x36 is printed on canvas, gloss lacquer spray and stretched on a strecher frame (
1600 ISO handheld night time image from my "Window Shopping" series) -
taken with Nikon D70 6MP
------------
24x36 is mounted to Masonite with ProTexture lacquer spray applied to the surface (
200 ISO 1/500th sync with wireless SB800 about 40 feet away from camera and to the left beside the subjects) -
taken with Nikon D70 6MP
-----------
One thing to remember is that the larger the print, the further away will be the viewing distance. And so even the way that the print will be displayed is relevant to the maximum size that you may want to print. A couple of years ago, I had access to ab article about a commercial job where the photographer used a 1 1/2 MP image file to produce a alrge display image to cover the whole side of a transport trailer.