How many photos with 1 GB?

If it's for screen display, you can stick to the lowest quality that allows for 1600 photos (Low/Normal or Low represents the resolution and Normal the jpg compression).

However: At this quality level, the image resolution is 1728 x 1152 pixels, so you need to know the definition of your output device (screen) as well as its dimensions to determine if you can display your photos in full screen without loss of quality. For example, on a screen with a resolution of 91 ppi, it will be fine up to a diagonal of about 22 inches (or 55 cm); if the screen is much larger, the image will be pixelated; with a screen at 121 ppi, you won’t be able to go beyond 17 inches (43 cm), etc.