Consider one additional feature which affects price, and that is OEM Vs Retail. With OEM, the buyer will not get any tech support from Microsoft. Retail includes tech support. Needless to say, OEM is meant for system builders and re-sellers, and costs less to purchase.
I would say that like someone else said, for most people, Home Premium is sufficient. That way you get Aero, and get all the nice features, including media center, but you forego some of the higher end options. However, you might find free tools that will perform some of the other functions if you look around.