depends on the lens and image sensor. different 5MP cameras could capture different amounts of details.
e.g. for 7MP cameras, the ability of the camera to resolve lines at gradually higher resolutions differs:
http://www.dpreview.com/reviews/canonsd550/page10.asp
however, generally speaking, 5MP pics could make pretty decent A3 size prints. just don't look too close though at the fine details ;p