It's not a matter of you being able to have 30 hours of HD AND 200 hours of SD. The drives aren't split like that if that's what you're thinking.
The HD recordings are BIGGER than the SD recordings. As you record hours of HD programming you will see your available SD space go down rapidly. It's the same space.
All OTA is mpeg2, and the DVR will just records what it tunes in. Re-enconding to MPEG 4 would be too processor intesnsive for HD, and would bring a little old 722 to its knees for hours and hours for a 15 minute program.