I have much the same situation, except that one of my PCs runs W2KAS and has AD, DHCP, DNS, RRAS, the full monty running. You should have one machine running the Internet link and ICS. The others should be set to DHCP. If (as it seems) DHCP fails to work, you need to manually set IP addresses in the 192.168.x.y range. Doing this, you will need to manually insert the DNS settings supplied to your internet-connected machine. The default router will be the local (192.168.a.b) address of the internet machine.

My clients are on a 10/8 network; my server's local IP is 10.a.b.c. This latter address is registered as settings in DHCP as being DNS WINS et al. The server's second NIC is on a 192.168/16 network, which talks to a Smoothwall firewall. The firewall has one NIC on 192.168/16 and one NIC on my ISP's network.