How do I add an application server to an exsisting network

Our network presently includes our own hardware firewall appliance with our Windows 2003 Server configured and as Domain Controller, DHCP Server, DNS Server, and File Server. We have 10 client pc?s, 7 desktops and 3 laptops. Now we need to add vendor supplied Windows 2003 Server to run the vendors custom application as well as incorporate a Cisco PIX device that comes with the whole package. I?d like to hear any suggestions concerning integration and configuration.