This expression should do what you want. It outputs "HD" if height is greater than 650 and "SD" otherwise.
if(compare([Height],>,650),HD,SD)
You could make this a saved expression and use it in other places too. Or just use it in your thumbnail text as you said.
What I like is to see the exact height, but only if the video is "high resolution". So I look at the width as my key. If the width is 1000 or greater, then my thumbnail text shows the *height* in parenthesis.
For example, if I have a file that is 1280x720, my expression sees that 1280 is >= 1000, so it puts (720) in the thumbnail text next to the video title. But if my expression sees 720x480 (DVD resolution), then it knows the width is less than 1000, so it doesn't show anything special. So I know it's SD content.
I would show you my thumbnail text exact expression, but it's rather complicated, as I have figured out a weird way of formatting my thumbnail text as multi-line by automatically breaking up the video title into several pieces so that long titles are not truncated. The expression is pretty ugly, but it works.
Good luck customizing your display text!
Brian.