DaArtist:
You don't need a second nic, check the specs of the d-link built-in firewall whether its satisfactory to you. If not buy a firewall box. If you want to use software firewall then either you install it to server and each workstation or only on the server with NAT and a second NIC look at the post of Anzluvsmc. Problem with this setup, server is doing the routing instead the dlink which is design primarily for routing and besides if your server is down everything is down, nobody can go to internet. This is also true with your current setup, if your server is down nobody will dish out an ip's to your workstation when their lease expires. So it's also not bad to let the router do the DHCP. Now, you really have to decide what do you want.