Hey Sam,
your eyes are irritated by the HD-picture. SD-picture allways looks like you have a big blur on the footage – compared to your original HD-quality.
SD-DVD only allows 720×480 for NTSC or 720×576 resolution for PAL as eric discribed.
and television/broadcast is working with the same resolutions. if you playback a DVD on a computer, it is the same – but most software-players scaleing the picture to full screen, and using no good scaling-allgorithms.
you see there is no other way to go – if you would like to have better picture-quality on optical discs you need to go blu-ray.
cheers
danny