You still can play with that server and with ESXi. I just wouldn't use it as a firewall
If you use another box that thas 3 NIC's you can place your server in a DMZ.
NIC one is the external interface, NIC two the internal interface. The last NIC is used for the DMZ.
Because you have a single server machine in the DMZ you don't need a switch to connect it to the DMZ interface. Just a simple cross-over UTP cable.
Using VMware you then can configure a nameserver, ftp server, www server etc.