Myth of Megapixels

See David Pogues' excellent article on why more megapixels isn't necessarily a good thing.

In my experience, there's definitely a trade-off. I used the Nikon D-100 extensively, a 6MP camera, before getting the D-200, 10MP. Although the D-200 definitely has higher nominal resolution, it's optimal ISO speed is 100, lower than the 200 speed of the D-100. When I set the ISO for 400 on the D-200 for low-light or flash photography, there's noticeably more low-level noise in the pictures, owing to the inherent loss of signal when so many detectors are packed into one chip.

Of course, Pogue misses a subtle point: the real reason why pro photographers want more resolution is to be able to crop a picture and still end up with reasonably high-resolution result for publishing. If you start out with a 4MP picture, and crop out half of it, the resulting 2MP picture will look crappy when blown up. With 12MP in the latest top-of-the-line models, you can crop quite a bit before the results are unacceptable.