Do I get HD when using my Roku stick, or do I need a cable tv box for HD?

Every version of the Roku Stick (except the ancient 3500 Purple Stick) is capable of at least 1080p. But that doesn't mean the content you're watching is HD. So the answer is, it depends on what you're watching. Most online providers offer HD content, so there's plenty to see.

But no, you don't need a cable box to view HD content on an HDTV.