The Starter version you would likely never see unless you were in a developing country.
Home premium is ok for home users.
Pro allows joining Domains for business, encrypted drives, granulated network file sharing and backups on a network, Remote Desktop, and Windows XP mode.
Ultimate allows 35 languages, bit locker security.
You can do Live Upgrades within 32 bit offerings and within 64 bit offerings to any version. Just pay MS for the correct license application and they give you the code for that version.