I agree with JohnHorb(post #4)
I'd forget the DHCP relay, i think this could cause more trouble than it's worth.
Are you putting the Watchguard in for your own training, or to beef up your network security? Can you turn off the router part of your ADSL router/modem? I ask this because otherwise your going to have lots of fun with double NATing.
Edit: This explains double NATing better:
http://support.iprimus.com.au/index....517&Itemid=214