“K64″ is what Apple refers to as the 64-bit version of the kernel beginning with Snow Leopard. As an end user, you really should not worry about the bitness of the kernel. If your Apple computer is not booting into K64 by default, you don’t need it—unless, of course, you know that you need it. (Say, because you are a kernel developer or an otherwise system-level developer and want to test somethin