Yannis has a good suggestion: The [Dimensions] field holds the dimensions of the video frames. Well, for most types of video. For at least one type I'm aware of, [Dimensions] is always empty for some reason.
The issue with trying to use this data is that it's formatted as "width x height", so you kind of have to break it out to try to determine HD versus SD. I have a lot of SD DVD rips that show up as 720x480, but not all of them.
What might work better would be to survey your current collection by sorting by the dimensions field. You'll probably find that everything below a certain point is SD and everything above that point is HD. You might find some that are blank too and you'd want to investigate those separately.
Once you do this, you could make a new field to hold some sort of resolution information that would be helpful to you. Maybe just a string like "SD" or "HD". Whatever works for you. You'd need to fill out this field each time you imported more videos, or do this exercise again periodically.
Alternatively, you could try to figure out how to break out the [Dimensions] field in a meaningful way that would let you categorize your videos automatically.
Brian.