If you're using the Red/White/Yellow cable you're not even using any kind of HD and any other connection type will look massively better.
The next best quality wise (ignoring the scart option , which is the same as above but with a different connector) will be the Cable with 6 connectors, As Outofthi5world says, it has the Red/white (Sound) , Yellow (composite video, the worst quality of all) and then a Red/Blue/Green (RGB) cable. Using the RGB cable for video, and the Red/White for the sound it will provide a massive improvement over using the Yellow plug for Video. Next up is the VGA cable (same plug as a PC monitor) only marginally better than RGB and can be very dependant on your display, but again massively improved over that Yellow plug.
Top of the heap, HDMI, carries the audio too but not available on every older Xbox.
Obviously this all depends on what inputs your Display device has !