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So, it's about dimensions. If you give someone a full size 4000x3000 pixel image they have the capability to print a decent size at a reasonable high quality. If you only give them an 800x600 image then they are locked into an image which they can only print to a certain size. DPI information in the image is completely meaningless unless you print it.
Learn something new every day!
For the web, again as far as I know - It's about dimensions AND filesize. If EITHER the 4000x3000 file OR the 800x600 is saved as 100% jpeg ( yielding LESS compression in the final file), you'll get respectable results if you try to print from it. Of course, those file sizes will be large. The 800 pixel file might be 700k to 1 meg, the 4000 file could be 15-20 megs. Some people DO INDEED upload in that quality for the express purpose of allowing people to print the photo. But for the purposes of protecting your work by uploading lower resolution files, don't worry about the DPI just save the file at a medium to lowish jpeg.