Can You Really Notice The Difference Between Hdtv And A Regular Dvd?

My standard dvd player shows an amazing picture on my HDTV. I am getting an HD receiver next week and just wondering if the picture will be getting any sharper. I am told it will, but how is possible for the eyes to see anything sharper than a dvd?
For you people watching HDTV. . . Do you really see a difference between HDTV (HD source) and watching a regular DVD?