When megapixels matter (or not)...

Several months ago, I came across a page on the internet and I can't find it now. It charted whether or not another megapixel or two it was worth it (as in, really made a difference). For example, 3 megapixels is way better than 2, but 5 isn't much better than 4 (just making that up as I really don't know).

Any thoughts?

Thanks
Kim