My experience is that 12 MP is quite solid for A2, around 16x23". I'd say that 24MP is visibly better but sometimes the difference can be quite small. But it also depends much on technique.
I have a small writeup on the issue here: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/24-how-many-megapixels-do-we-need
BTW, I have a 70x100 cm (27x39") print on my wall and it is reasonable sharp at 1m viewing distance!
I just viewed the C2P module on mp size and print size and was wondering how interpolation affected what was said. Michael indicated that a 10mp camera should only print up to about 13 x 19. I have done quite a few up to 20 x 30 that really look good. I have also done some cropped canvas at sizes like 14 x40 that have won some awards. I shoot with a Nikon D200 and usually under iso640. Michael and Jeff seemed to be indicating that you have to have larger mp cameras. How cast in stone is this opinion?
By the way, the videos are excellent. Can't wait for more.