I decided not to use AHCI because it showed my hard drives removable along with USB flash drives and I did not like that. They say AHCI might be good for the larger hard drives, but may not be enough for you to try it.
It is an interesting question, though. If the install can't read the IDE drives to validate the Vista install, will it activate...
Not sure, but if you feel like doing a registry hack, you can change the current drive setup to AHCI. If you were to do that, the question would no longer be relevant.