If you running ICS on Win 2000, Mandrake can use it as long as the IP Adress is a private address i.e 192.168.0.X with a subnet mask of 255.255.255.0 and the default gateway should point to the Win2000 box, which should be 192.168.0.1 by default.
All this can be configured in /etc/sysconfig/network-scripts/ifcfg-eth0
ICS makes the host 192.168.0.1 by default
With the subnet mask 255.255.255.0
You will also need to setup the DNS servers you use in /etc/resolve.conf
ICS also runs a litle dhcp server as well, but i dont really know what this is capable of? You could try getting an address dynamically and see what happens?
If I were u I would blow away the windows install on the host and install Linux. Then use iptables masquerading to share the internet connection and iptables filtering to act as a firewall. Use dhcpd to offer IP addresses to the clients and use squid to be the proxy server.