The Megapixel Myth
If you're like most people, when you compare digital cameras, your eye probably goes first to the megapixel count. It's understandable - the manufacturers and the press generally grasp this number close, waving it as though it proved something. And it does, in a limited way. The megapixel count tells you, more or less, how large an image you can print out at a 300dpi resolution and get really good detail. (It actually gets a bit more complex, as you can enlarge images, but let's not go there for now. I'll cover enlarging images - something easy to do - in some future post.)
You can work in two different directions. One is to check the pixel dimensions of the sensor. A camera like the Canon 10D (an "older" model, but one that I still use) might have 6.3 megapixels in a sensor that is 3088 pixels by 2056 pixels. Remember that we want 300 dots per inch for good print resolution. Divide 3088 by 300 and you get about 10.3 inches; divide 2056 by 300, and the result is about 6.9 inches. So you can figure that is the largest native print size.
The other direction is to take the actual print size you'll want - take 11x14 inches as an example. Multiple each side by 300 pixels per inch to get 3300 x 4200, or 13,860,000 pixels. That's close to 14 megapixels.
But the qualify of the image is affected by other factors, the largest being the quality of the lens and the software you use to fill in the image. Software? Yup, software. From an imaging standpoint, those sensors still have huge gaps between the pixels, and each pixel only sees one color, Something has to figure what goes between the pretty colored dots. Cameras can vary greatly in the quality of either the software or the lenses.
If you want to know how the actual images compare, you need help. Reviews can be good if the people writing them actually can tell the difference between one set of results and another. One good source for opinion is PC Magazine camera reviews, generally written by Daniel Grotta and Sally Wiener Grotta, two colleagues of mine who know a lot about the subject from all angles. Daniel also has his DigitalBenchmarks site, which has some free detailed recent reviews.
If you want to get more for your money, consider an older model camera. When new models come out, stores will be looking to clear their inventories. Getting last year's model is generally going to be safe and will give you plenty of power for what you need to do.