Attention: TDR Forum Junkies 
First, a definition: as used below, a pixel is one picture element that contains all three primary colors. It is *not* a picture element that contains *one* of the three primary colors.
An 8x10 print at 300x300 resolution requires 7,200,000 pixels. A 2,100,000 pixel image is good for about a 4x6 print at 300x300 resolution.
For a good quality print, you don't want to go less than 300x300. Also, you don't want to have to 'scale up' the picture; image scaling algorithms generally work best when scaling down in size. The experts say you should take a picture at at *least* the resolution you'll use to display it. That said, a 10%-20% increased image scale probably won't be noticeable.
A 3,100,000 pixel camera will be good for about 5x7 prints; they'll still look pretty good scaled up to, perhaps, 6x8.
If it is a shop telling you that 2. 1 MP images will print *really* well on an ink jet photo printer at 8x10size, take one of their cameras, take a couple pictures that have sharp details in them, and have them print the pictures, un-retouched, at 8x10 size using standard, consumer-quality software and equipment.
Then take a jeweler's loupe or magnifying glass and examine the picture. If you see fuzzies, anomalies, inconsistencies and other 'defects' in the print using the magnifier, you can be certain your eyes and brain will register them as well when not using magnification, even though you may not really be *conscious* of those 'defects'.
In essence, you'll be thinking the picture just doesn't look as good as you'd like, but you can't explain why.
I hope I haven't further confused you.
Fest3er
