Microsoft has listed all 200 or so games they say that the Xbox 360 will be backwards compatible with, as well as revealing that all such games, not just Halo and Halo 2, will be automatically upscaled to 720p. Microsoft's VP for the Xbox Product Group, Todd Holmdahl, also did a Q&A about the 360's backwards compatibility.

As mentioned earlier, the hard drive accessory is necessary for backwards compatibility, in order to store the "emulation profile" for the game you're trying to play. The hard drive comes preloaded with early versions of the profiles for Halo and Halo 2.

NOTE: Marty "the Elder" O'Donnell warns that this early version of the emulation profile for Halo and Halo 2 has serious problems with audio, and urges everyone to get the new version via XBL or Xbox.com before playing:

Without the update the audio will suck. Without the update you will think that your new Xbox 360 is broken.

Other profiles can be downloaded from XBL or installed from a CD, either one you burn yourself after downloading profiles from Xbox.com on your computer, or on a CD ordered from Microsoft.

Saved games will not be transferred from old Xboxes.

This week's Bungie Update mentions that texture pop-in on Halo 2 should be "slightly improved" due to the new box's DVD drive. Copies of Halo and/or Halo 2 running in emulation on Xbox 360s are compatible in network games with players running those games on original Xboxes. Halo 1 still doesn't support Xbox Live, and won't. The update also has word on new matchmaking playlists.