(pc) monitors can only render up to 72 dpi. 300+ is used for printing. however, i've heard that macs can see up to 91 dpi but i'm not sure if that's true.
do this. open up photoshop or fireworks (i'm using fireworks right now since i dont feel like opening photoshop) and make a new image 100px x 100px @ 300dpi. then go to "Image size..." and uncheck "resample image". now change the dpi to 72. you should see the size of the image change from .333in to 1.389in. that's what i meant. it is still the same amount of pixels (there's 10,000 pixels in each image) its just how far they are spaced apart is different. if you print both images, the sizes and quality (although it might not be too noticable) will be different. but when viewing on screen, you don't see the 300dpi, only a 72 dpi rendition of it.