There is install restrictions on a OEM and Retail version but I can't remember off the top of my head what they are.

Home, Pro, and MCE all are different in the sense that Home doesn't come with the ability to log onto domains and things like RDP installed by default. MCE is basically a program addon to XP Pro that can be run independently or upon startup; There is a hack to get XP Pro to work with MCE if you look hard enough.

OEM is perfectly fine for most people.