A good rule of thumb is to purchase a camera with the highest number of megapixels you can afford.
The more megapixels the camera reproduces, the better it will capture details.
Megapixels determine the quality of the image resolution.
If, however, you actually plan to print out and enlarge photographs that you take with your camera phone, find one with at least 3 megapixels, although these models are few and far between at this point in time.
Basically, higher megapixels mean better quality images.