Other cameras will go down lower to 640 x 480 pixels, requiring even less memory. However, the lower the resolution, the less detail your photographs will capture. 800 x 600 pixels is perfect. JPEG compression is another factor. When a digital camera stores a photograph in memory, it compresses it down so that it uses up less memory.
More compression requires less memory.
However, more compression causes a degredation of the image. Memory External memory cards are used to extend the camera's internal memory. The more memory available, the more photographs can be stored. A lot of cameras have 8Mb of internal memory, a lot more today have 16Mb. Most all cameras support external memory.