Don't know about the FTP thing, but the difference between Administrators and Power Users is this:
Forest Admin can do everything and anything on every server, domain admin can do so to their domain, etc. Full Control, basically. They can mess with EVERYTHING. (Bwhahahahaha. POWER TRIP!!) @=)
Power Users do not have Full Control over everything. They can create users and groups in whatever local group they have those privledges in, but can only manage what they create (not what someone else created) and can only manage those network shares and printers they create.
IE: If Bob the Power User creates Network Share 1, Printer 2 and Group ABC, then Anne the Power User cannot mess with them without Bob's login info, which is a a major security risk. On the other hand, Terry the Admin guy can mess with both Bob's and Anne's stuff because he has control over everything in that domain (presumably).
That's the way I understand it, anyway. From the information I have, TheGoodRev is correct. Power Users is a default group on local environments (check your member servers), not domains.
You can use the MMC snap-in Local Users and Groups on your member server to manage the Power Users OU.
Hope that helps. @=)
Catadmin - New to Server Admin, but willing to learn... All help is appreciated.