While there's definitely a visible difference between 16 and 32 bit color, i doubt that there will ever be consumer level cards with 64 bit color. 24 bits is enough to produce more colors than the ...
16-bit color usually means a 565 RGB pixel format (5 bits red, 6 bits green, and 5 bits blue). 32-bit color means you get a 8888 RGBA pixel format (8 bits red, 8 bits green, 8 bits blue, and 8 bits ...