Update on my hosting situation:

I am able to host with my Sygate firewall off, and DMZ enabled.

I am now able to login into the foyer with DMZ disabled and router port-forwarding active, with my firewall turned off. I have not had the opportunity to test hosting under these conditions. Hopefully I'll get to test that tonight.

I get "Account Details Incorrect" when trying to log in with my firewall on. This suggests issues with Gamespy port forwarding in my firewall settings. My guess is once I figure out what's missing here, I will get to the foyer. If I make it that far, I think it will be easy to deal with any remaining issues and be able to host with specific router port forwarding (DMZ disabled), and with my firewall on but also forwarding the correct ports. This is my goal.