You seem to be missing the point here. iPhoto store the photograph exactly as the camera recorded it. The important part is not dpi but number of pixels. If you camera records, for example, 4000 X 3000 pixels that is exactly what iPhoto stores. If you shoot jpg it stores jpg, if you shoot RAW it store RAW. iPhoto does not change or reduce the quality in any way.
DPI only becomes relevant when the photo is viewed or printed so, an example with an over simplification, if your camera produces 4000 dots horizontal and your printer can resolve (print) 100 dots per inch horizontal you could print a picture with a width of 4000 / 100 = 40 inches wide before losing resolution. If you had a printer (never seen one) capable of 1000 dots per inch then 4000 /1000 = 4 inches so you would get an amazingly sharp picture but only at 4 inches wide.
When your computer screen shows a picture its size is defined by the resolution of the sceen so if the screen resolves 74 pixels/dots and each dot in the picture = one screen dot a 4000 dot picture will show on screen 53.3 inches wide. If the screen resolves 96 dots it will be 41.6 inches wide. In practice programs such as iPhoto will show it on whatever screen real estate is available but they are just designed for viewing and basic quality control. True editing software such as Photoshop allows you to zoom into individual pixels to edit them if needed but iPhoto still stores the full sized image and the ability of photoshop or similar to edit is the same whether the shot is dragged to the desktop from the original SD or imported into iPhoto.