What does HD-DVD stand for?
Most people think "High Definition Digital Video Disc" when actually it's "High
Density Digital
Versatile Disc" again it can store ANYTHING, just MORE of it
As far as HD content available today
-OTA or Over The Air: Broadcast HDTV available in most densely populated area, offers lots of free HD content if you have an appropriate Receiver (be it built in or stand alone)
-Cable/Satellite Feeds: Pay your provider about an extra $60 and you too can get a whole 5 or 6 extra channels in HD
-HTPCs Most PCs can output in HDTV resolutions, Most will have VGA or DVI outputs but a few mobos built for HTPC use will have Component outputs as well. You can do your typical computing, play games, and play WMV-HD videos, which can be either downloaded (or purchased on DVD discs)
http://www.wmvhd.com If you're wondering YES the 360 will be able to play these discs, Yes they're typically split up over multiple discs (because DVDs are too small for the amount of content) and NO there really arn't any good movies available in this format (Terminator 2 is the only Hollywood movie, everything else available is a science documentary).
-WMV HD players, hitting the market this holiday, stand alone players for WMV HD discs (hopefully we'll get some good movies along with it)
-Up-Scaling DVD players, They take your ED movies and upscale the picture into HD. They're a bit more expensive than normal DVD players but definitely worth it.
-Your Xbox There are about 20 something games that can produce true HD graphics, Most have kind of crappy graphics to begin with but there are a few gems (Soul Caliber 2, Amped 2, THPS4+, True Crimes, Xmen Legends, etc). As for DVD playback on your Xbox, using XBMC or Avalaunch as your DVD player they can upscale your DVD to HD 720p, though I've heard that they have trouble sometimes with HD 1080i.