PDC and BDC went away with NT. All domain controllers are writable in server 2000/2003. What I would recommend as a starting point, is to put domain controllers in each location. On one domain controller in each location, you would make it a global catalog (GC). You would also make sites and associate the each subnet with the appropriate site. You would configure you DHCP in each site to issue the appropriate IP, subnet, DNS, WINS (is used) etc.
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