The 400D uses the DIGIC II image processor, as is used in the 350D. The 400D file numbering system holds 9,999 pictures, as opposed to 100 photos in one folder with the 350D.
The 400D's file numbering system is more extensive than the 350D's, allowing for storing up to 9,999 images in one folder compared to the 350D's capacity of 100 photos.