Frame-buffer size for each of the systems is
640 × 480 × 12 bits ÷ 8 bits per byte = 450 KB
1280 × 1024 × 12 bits ÷ 8 bits per byte = 1920 KB
2560 × 2048 × 12 bits ÷ 8 bits per byte = 7680 KB
For 24 bits of storage per pixel, each of the above values is doubled.
900 KB & 3840 KB & 15360 KB