DPI stands for Dots Per Inch and only matters for printing. For screen display dpi is irrelevant and the size of an image onscreen is determined only by how many pixels it is comprised of, along with the size of the monitor and the resolution (damn that word!) of the end users display.
As for memory usage, note that the filesize of an image is also determined by it's pixel dimensions. The dpi setting is just one little number tagged in the file somewhere.
To verify this, just save two images with the same pixel dimensions but different resolutions and observe both their filesize and appearance on-screen. They will be exactly the same; now if you print the images things will be different.
Note that Photoshop muddies the issue by automatically changing the pixel dimensions when you change the dpi, so in the Image Size dialog you have to first change the dpi and then change the pixel dimensions back to what they were originally.
As for memory usage, note that the filesize of an image is also determined by it's pixel dimensions. The dpi setting is just one little number tagged in the file somewhere.
To verify this, just save two images with the same pixel dimensions but different resolutions and observe both their filesize and appearance on-screen. They will be exactly the same; now if you print the images things will be different.
Note that Photoshop muddies the issue by automatically changing the pixel dimensions when you change the dpi, so in the Image Size dialog you have to first change the dpi and then change the pixel dimensions back to what they were originally.