I AM FAIRLY NEW TO DIGITAL PHOTOGRAPHY, SO BEAR WITH ME. I DO NOT UNDERSTAND ALL THIS ABOUT MEGAPIXELS AND RESOLUTION IN THE LIGHT OF INTERPOLATION SOFTWARE (WHICH I HAVE NEVER USED). IF IT DOES WHAT IT ADVERTISES, ENLARGES UP TO POSTER SIZE WITHOUT LOSS OF RESOLUTION, THEN WHY BUY EXPENSIVE CAMERAS? 10 MEGAPIXELS SHOULD BE ENOUGH FOR ANYTHING REGARDLESS OF SENSOR SIZE. DOES THE SOFTWARE REALLY DO THIS?
If you have a camera with 5 megapixels the amount of image information would be half that of what a 10 megapixel camera would record. Interpolation does not add any information, it just makes more pixels by division of the pixels in the original, duplicates of the information that is there already, but the same information. It is like making a photo print on a rubber sheet and then stretching the rubber to make it a bigger image.
In terms of image quality with digital cameras resolution in megapixels is everything as it is the measure of how much image information is gathered, and interpolation is just a way to expand that same information laterally. The old analog film parallel is that a medium format 120 camera will record more information (quality) than a 35mm's smaller film format. But of course you can make a 16x20 enlargement from both a medium format 120 film frame or a 35mm, but the quality of the 16x20 from a 35mm will be inferior to the one enlarged from the medium format 120 film.
PS please don't use caps to post messages, it is considered in poor taste, like shouting when talking in a normal voice is appropriate.