Setup of pFsense on established netowrk as subnet

[dmt]

I have several computers that are part of a existing network and I would like to have pFsense manage those computers as my personal network.

The way it would have to work:

DSL Modem > DLink router > pfSense > Computer

The DLink router currently handles DHCP for the rest of the network.

The computer that I have installed pFsense is an old Dell pIII 450 and currently has 2 NIC's. I will add a few more once I can make this work properly.