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The term Dots Per Inch (DPI) causes lots of confusion. Try to search Internet or UseNet groups, and you will see lots of discussions around this topic.
First and foremost, forget about DPI if the image don't touch a printer!! The DPI setting doesn't affect screen display of the file. The screen display is only controlled by the pixel dimension of the image (such as 640 x 480 pixels, 1600 x 1200 pixels and so on), and it's finally the screen resolution that determine how large the image will look.
What DPI do control is at what density the image is printed.
To attempt to explain the topic, let's assume we have an image which is 720 x 540 pixels. Note that the width of 720 pixels aren't completely taken out the air! This is because our math will be so much simpler!!
On your screen, this image will be exactly 720 pixels wide, so if you have an traditional LCD screen with 1024 x 768 pixels resolution, this image will occupy approximately 2/3 of your screen. Anyone with 1600 x 1200 displays around ? These screens will show the same image in approximately 1/2 the screen. And the cool thing is; The displayed image will be the same size no matter if the DPI is 1, 72, 96 or 300! So much for the screen display.
When it comes to printing, the DPI start to work. Note that the DPI is Dots Per Inch. This means how may dots the image contain per inch. If the DPI for example is 72, then the image will print with a size of 10 inches (720 / 72 = 10). If the image has 144 DPI, the image will print with a size of 5 inches (720 / 144 = 5). Note that the image detail isn't changed at all, only how many pixels we cram into each inch !!