From what I can tell yes. There's three HD flat panel TVs in my home and we have dish network. 2 TVs are connected to HD digital box while one isn't. The one that is not connected to an HD digital box have standard definition. The difference are very noticeable. Also you can try connecting a device that have a hdmi port to your TV and see if you notice the difference.
You can use component cables for HD signal, but not 1080p, it's just not as good and may have some progressive scan issues and maybe a few image issues depending on the devices used. I have an HD cable box with no HDMI, component signal shows up at 1080i on my TV.