> I wonder where the x86-64, x86_64, and (the most strange of all) x64 names came from.
The "x86-64" name is the original one, and came from AMD themselves: https://web.archive.org/web/20000817071303/http://www.amd.co... (and "x86_64" is obviously an alias for where a hyphen is not an allowed character, like identifiers on many programming languages).
The "x64" name came from Microsoft, probably due to file name length limitations (this was before Windows XP unified the Windows 9x and Windows NT lines).
IIRC, the "AMD64" name came later, probably to distinguish it better from Intel's IA-64 (Itanium).
I think we should call it AMD64, since that's what the people who actually designed it wanted it to be called.