S-Video will not support 720P and 1080I HD signals. It may look pretty good, but it doesn't look fabulous. Only Y, Pr, Pb component or DVI/HDMI will support true HD formats. If you are watching them from a regular cable box it's not ED/HD either. You would need to take the coax straight to your cable input on the set and let your set's digital tuner auto-tune them. The true digital ED/HD signals won't be on familiar channels. They might be something like 52.1 or 59.2. Only primetime HD programming and true HD sport shows will fill the screen. All other ED programming will have black bars because it's 4:3 format, but will still look much better than standard NTSC VHF we used to watch.
Don't expect the ED/HD network channels to be on regular cable for long. The cable company will scramble them soon for decoding with a cable box or CableCard. Enjoy while you can. Then, use a good UHF antenna to pull in free off-air ED/HD digital network signals. (air input on set)
No sat company can provide ED/HD networks like ABC, NBC, CBS, Fox, etc. yet. Direct TV has two new birds in orbit for rollout of mpeg 4 and future network HD for 2006-2007.
Sat companies do have Discovery HD, (excellent channel) HDNet, (another excellent channel) ESPN HD 1 & 2, HBO HD (1 channel) , and Showtime HD (1 channel).