First we need to make a an important assumption. We're going to assume our photos fill frame, so there's no need to crop. With this assumption let's look at how many megapixels are required.

If the image will only be seen on a typical computer or TV, no more than 2 megapixels are needed. For a 30 inch monitor run it at 2560x1600) resolution at most 4 megapixels are needed for wallpaper.

If the image will be printed as a 4x6 or 5x7 snapshot, about 2 megapixels are needed. For an 8x10 about 5megapixels is plenty.

With a really sharp image, i.e. no subject or camera movement, 11x17 enlargements are easily achieved on a 6 megapixel camera.

For the average person anything more than 6 megapixels is overkill. Don't pay extra for more pixels!

For folks who want really large prints, 6 megapixels isn't enough. If a shot is sharp and properly processed for printing, a 12 megapixel camera picture can readily produce a 30x40 print. Here's a shot of the Cape of Good Hope taken with a 12 megapixel Nikon D200. It's stunning at 30x40.