Small Network Setup

I have a PowerEdge1800, 5 pcs, 1 firewall, 1 router, 1 printer, 6 hardwired cat5 connections to workstations in the office. Can someone tell if I would be needing a switch? Is this enough Hardware to build this small network. Also, Can someone please tell me what the heirarchy should be (like ISP,Firewall,Router to server and wall connections) or (like ISP,Firewall,Switch to Server and wall connections). What is the standard practice? Thanks in anticipation.