Bungie has posted an
FAQ that covers most of the questions about the upcoming PC port of
Halo 2. It covers most of what you wanted to know but doesn't answer the question I have which is why a two year old console game requires a cutting edge operating system to run (especially since they aren't using Direct X 10). Snarky comments aside it is a good read since it looks like they will be using Live Anywhere and will support Live Achievements.
