1st question – WHY??
Subnet, Supernet, and community – 520 computers in one address space would be subnet 255.255.255.252 for 1023 available. Does every workstation need access to to every server? Are all the servers in the same physical location? How have you configured you outside (internet) access? Is your firewall an appliance, or a program on one of the servers?
I will presume that we need to separate the discussion between physical and logical networks. If all the servers and the firewall are physically colocated then attaching all to the Gigabyte switch is straight foraward and there is no change to IP addressing. If you wish to segregate workstations and server sets by different subnets then you are calking VLAN (Virtual or logical networking) and CISCO and Nortel have several hundred whitepapers you can download.
In either case – WRITE IT DOWN before you start. Get administrative support for the plan of action and let them deal with the complaints when the odd man out finds he no longer can access everything.