Newbie help - connecting server to existing network

[mossc001]

I want to connect my server to my existing router along with the 3 normal office computers, although the server will be a web server. Rather than using DMZ I will just use port forwarding. As one of the computers on the network is accessible from outside the LAN does this pose a threat to the other computers in the network?

So basically would it be advisible to connect my server to the firewall router and use port forwarding, then connect a VPN/Firewall switch to the router (where the server is connected) and connect the office computers to this?

Any help would be much appriciated!