Home is the most basic. Professional includes "Windows XP Mode", which I believe is an overglorified compatability mode. Company network advancements, and you can save backup and restore points to a network (where as with Home it is just to the computer). Ultimate includes all the above with the BitLocker which is some encyption thing (know little about it) and the full language compatability.
For most users Home will be quite adequate. If money were truly no object I would say Professional might be worth it for the "Windows XP Mode" (though I don't know, Microsoft says this is mostly for small-medium businesses, so maybe compatibility mode still works fine?).
I went for Ultimate because I like the language support as I occassionaly enjoy using some Russian now and then
*- I have a friend who works at Microsoft picking it up for me, so no, I have not installed it yet.