Changing Active Directory Domains

Hoping someone can help with a Domain change.  I'm overwhelmed and can't think clearly.

I need to merge (sort of) two separate domains.

Domain 1 is:
     - using SBS (which I want to do away with)
     - with the domain name of citylan
     - with the scope of 172.16.3.XXX
     
Domain 2 is
     - using server 2003
     - with the domain name of annexlan
     - with the scope of 192.168.1.XXX

I want to end up with (I think):
    - Forest of citylan
    - Sites for chlan (changing everyone currently on citylan to this one) and annexlan
    - Everyone using 192.168.1.XXX except for a few computers that need to remain on     172.16.3.XXX


Everyone can use the same gateway except for a few computers.  I currently have two routers/firewalls on each network and there is fiber running between each site.
Just need sites to be able to share resources yet be restricted from full access to other sites computers.

Losing connectivity has to be minimal as this is a 24/7 operation.

I'm not sure if I need to get a third server and build it this way and manually enter each user (ugh) or raise a domain level and merge the other....

Any help is super appreciated!