A SATA II (300) single drive inside your Mac won't be any faster than a SATA I (150) drive. So just go with whatever is cheaper and the size you need. Either one should work fine, although, some people have had a few issues with the earlier WD drives, so i might avoid them and go with a Hitachi or Maxtor. Your Mac's controller is SATA I (150) by the way, but that's not the reason a SATA II (300) drive won't run faster. It has to do with the limits of the drive mechanism itself.