Help with my Sanyo HDTV, blurry picture?

Ok I understand a HDTV will not provide HD picture quality unless I am getting a signal. But does that go the same for my Xbox games? The quality is horrible! It's all blurry and kinda distorted. I understand Xbox makes a HDMI Cable that you can use to get HD. But HD should work with dvd's and other game consols right? I mean nothing is in HD.My Emerson tube TV that I just got rid of works better than my New Sanyo flatscreen. And my Emerson from 2001. Any tips?