most everything i look at says you need between 200 and 300 pixels per inch to make quality prints, so i would guess that 300 would be an optimum number, and 200 would still yield decent results, but you probably shouldnt go any lower than that.
other things ive seen people discuss is viewing distance. if the print isnt something that people will be viewing mostly at a close up level, you can print larger than normal, but if its going into a photo album, you want as many pixels per inch as possible.
i havent had anything printed larger thn 8x10 with my 8MP camera, and i never had anything printed from my 5MP camera, so i cant give you an opinion on what a 5MP might look like at a certain print size. im sure someone else will be along soon to give you more, and better information.