Many people (including marketing departments) seem to be looking at cameras almost purely in terms of megapixels. Is this a valid approach? The sensor is efectively the "film" in a digital camera and the number of megapixels is equivelent to the grain in film and who wants grainy pictures? But how "grainy" is a low megapixel image and would you notice in the way you use images?
Certainly if you make very large prints you will choose a high megapixel count, but do you do that? 
I decided to take test images with my three camera bodies and show you the results. I used a Sigma 24-70 lens at an indicated 35mm focal length (about 50mm equivelent for the APS sensor size) and all three bodies set at ISO 200, F8 and 1/30th of a second in manual mode. Fot the first test I made JPEG files of average size and using the full frame without any sharpening or other adjustments in Photoshop:-

Pentax Ist 6 megapixels

Pentax K10 10 megapixels

Pentax K20 14.6 megapixels

The three cameras do of course have other differences, the K10 and K20 for instance, can shoot in an automatic mode where the user sets shutter and aperture and the camera adjusts ISO speed to get a correct exposure. If buying a compact camera the lens offered may vary widely between models. 

Pentax K10 10 megapixels

Pentax K20 14.6 megapixels