Even JPEG Fine is ok. Just upsample the image and perform some unsharp mask. You're not going to look at an A1 poster 40cm away, so you won't be able to see the details. Even printing at 150-200dpi is acceptable for that size.
actually hor, just do it to the highest res you can,
try not to do the text and other design elements in Photoshop (resolution dependent)
use illustrator or indesign (any other vector based software also can)
Thinking of which, I have printed posters 1m by 1.5m before with a 8 megapixel camera too -- it's just a 30D, but well the picture's alright especially if you view from a distance... The details are there and of a somewhat acceptable quality, but don't expect the micro-details to be there.. like your skin texture and all.. afterall you are upsizing it a lot..