I'm in the process of buying a Dell Poweredge 1600sc server with windows 2003 server small business premium.  Since I'm building the network from the ground up, can someone give me some advice on how to set up the connection/protection aspect of my network?

Known Facts:

Network has 4 pc's and the 1600sc server.  All will be connected to a gigabit switch.  Also connected to the gigabit switch will be a PowerConnect 2124 thru its gigabit port.  This device has an additional 18pc's that do not need to access the 1600sc server.  (They access one of the four pc's that is dedicated to running a web-server program called "Resin")

Unknown:  I'm thinking about getting one of the Watchguard Fireboxes (soho 6tc) to serve as my internet firewall.  What is the relationship between the Firebox and Windows Server 2003 running ISA?  Do I need both? Should I have both?  Do I need one nic card or two for the server?  I don't have a networking specialist on site, so I'm really looking at this from the perspective of easiest to configure and maintain while also providing adequate security.  I've setup plenty of linksys routers for home networks and internet sharing, and based on what I've seen, the Firebox doesn't look too daunting.  ISA on the other hand, well, lets just say I hope it comes with a complete setup wizard... 

Any and all questions, suggestions or help would be greatly appreciated!