Obviously, if your 400MHz is a Lombard or Pismo, the answer is no. If your 400Mhz is a G4, I don't see why not, but you've got your work cut out for you. (I mean, this is how the computers are built. They have the case, and then they put different options/configurations in the same case.) You may want to check in the hack your mac forum, though.
Also, you can make your signature not part of your post in the User CP.