Don't forget to pay attention on one little thing... most of antivirus software has workstation(personal) and server(enterprise) version. Most of the time personal version can't be instaled on server...

I don't have much experience in this area so I don't have much more info to add. I personaly use Symantec Antivirus Enterprise together with derfeeld firewall and Spybod S&D. But my network is only 10 computers...