Unfortunately the answer depends on your TV. Some TVs you can see an slight improvement with the DVI cable. The difference is sending your TV a digital vs analog signal. And yes you can receive all your programming HD and non over the DVI cable.
My cable box has both component video and DVI connections. If I just use a DVI cable, will I be able to watch non-HD programming? (I know that I still need an audio cable; I'm just asking about the video signal.) Or do I still need to use a component cable for the non-HD channels? Also, should I just use component cable for both HD and non-HD? (In other words, does using DVI make HD that much better?) I tried asking my cable provider's "technical support" people, but they did not understand my question.