this article in the new york times blog explains exactly why you shouldn’t be only looking at the number of megapixels on your new digital camera, through an interesting experiment:
On the show, we did a test. We blew up a photograph to 16 x 24 inches at a professional photo lab. One print had 13-megapixel resolution; one had 8; the third had 5. Same exact photo, down-rezzed twice, all three printed at the same poster size. I wanted to hang them all on a wall in Times Square and challenge passersby to see if they could tell the difference.
Even the technician at the photo lab told me that I was crazy, that there’d be a huge difference between 5 megapixels and 13.
I’m prepared to give away the punch line of this segment, because hey—the show doesn’t air till February, and you’ll have forgotten all about what you read here today, right?
Anyway, we ran the test for about 45 minutes. Dozens of people stopped to take the test; a little crowd gathered. About 95 percent of the volunteers gave up, announcing that there was no possible way to tell the difference, even when mashing their faces right up against the prints. A handful of them attempted guesses—but were wrong. Only one person correctly ranked the prints in megapixel order, although (a) she was a photography professor, and (b) I believe she just got lucky.
(…)
In THEORY, you should be able to see a difference. But you can’t.”
exactly. besides, more megapixels mean bigger files, for equivalent quality. focus on the lens quality and the image sensors behind it, and start spending the rest of the money on trips to the places you want to photograph instead.