One small note on updates. If you have your registrar host it, you can
still make any updates at will very easily so that should not dissuade you
from using a registrar. That said, if you still want to dedicate two DNS
servers for public dns, then I would not install AD. Just setup two servers
with NT, w2k, or w2k3 (or linux with BIND for that matter) - install DNS and
create a primary zone on one and a secondary zone on the other. Setup your
domain (nt or AD) inside your private network with no connection to dns
servers in the dmz (I assume you will have a firewall separating the public
and private networks.)