You may not see much difference on a small size screen.
Have you ever wandered what’s the big deal about expensive digital cameras because your cheap point-and-shoot camera produces the same quality photos when viewed in an email attachment? If all you’re going to do is email small size photos, a cheap camera will do a decent job. However, if you decide to do an 8×10 enlargement of the same image you will see many shortcomings when compared to a 6 megapixel camera.
The exact same “science” applies to high def. When you compare SD originated image to HD image on a small size monitor you may not be able to appreciate the amount of information contained within the HD signal. The difference becomes more noticeable with increase in TV size. At about 28″ diagonal most people will react favorably to HD.
TV manufacturers have responded to the trend of larger screen size which has been particularly accelerated by the drop in prices of large screen LCD monitors and TVs. Average TV screen size in the U.S. is 38″.