Need advice on upgrading firewall.

I am presently running Gauntlet 3.2 on a Unix box that is also our DNS and Proxy server to the Internet. We are using the NT platform and are upgrading to W2k and will be going to Active Directory sometime in the next 12 months, I want to upgrade our firewall but have no idea what we should go with. Should I stay with Gauntlet or go with a different system entirely? Any suggestions would be appreaciated