Sorry, I don't know hardly anything about Macs and/or OS X.
That said, OS X is a variant 'nix based upon FreeBSD. I can boot FreeBSD from two floppies (OS not installed) and connect to any Windows IP based address.
If you have at least one NIC in your Win2000 PC, you can implement ICS in conjuction with a modem.
Then I would just use a crossover cable (not a straight through cable) between both PC and Mac (if you have no hub/switch).
In Windows 2000, you find the Network and Dial-Up connectins and the dial-up settings for the modem. In the modem properties, select the Sharing tab and Enable Internet Connection Sharing for this connection, then select your NIC (or appropriate NIC if you have more than one). It will come up with a message telling you that the IP address will be changed on this NIC. Select OK. Now check the IP address (ipconfig /all) and you should see that it is now 192.168.0.1 with a netmask of 255.255.255.0
This is standard on all ICS. ICS implements a stripped down DHCP which can't be configured, so either your Mac rig needs to be able to accept dynamic assignments from the Win2000 ICS shared connection or set the Mac rig with static, manual addresses.
Assign IP addresses and network masks to the Mac rig which work with ICS from the 2000 PC. Set the gateway on the Mac rig to be 192.168.0.1 (the Win2K PC NIC). ICS will use the range 192.168.0.xxx (xxx cannot be 1 or 0) and you should select a higher IP number like 248, because these numbers will not usually be assigned by ICS except if there are many more users using ICS. The subnet mask is usually 255.255.255.0 and must be the same on both NIC's that are interconnected by crossover cable.
I don't know what other things you need to setup on your Mac rig, but this is where you should probably start.
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