By their very nature, DVD's only deliver 480 horizontal lines of resolution. HD has either 720 or 1080. When they say that the DVD was encoded from HD masters, they don't mean that the picture you will see will be HD. That's just a gimmick to get you to buy the thing.
You're wondering if you need to buy higher end cables? If you have an upconverting DVD player, go for HDMI. The player will artificially enhance the image to give the appearance of HD, though it won't be as good as true HD.