If you are planning on using Active directory then you need at least one machine to use as a domain controller as well as a network switch.
If you aren’t going to bother with Active Directory and you’ll simply create accounts on the workstations then you just need a network switch to tie them together.
Use SBS (Small Business Server) if you can afford the lic. Its has everything Windows 2003 Exchange 2003 ISA (depending on version) you wont go wrong with this. What type of Inetrnet connection will you have? Personally go 4 a Cisco 800 series router cheap and very good – will handle internet with ease. Switch Netgear 24 Port allows for growth. Bang your done! But as per usual it all depends on your budget.