Server PC- To start off with I'm assuming you are using the win2k PC as the server. Unless you have 2 IP addresses, you'll need to make sure you have then modem connected to the host PC if you can. If you have an internal modem I'm not sure what to do, I do believe you might need to run a proxy server (software).
I can help you with setting up an external modem. Make sure the your network card you are using to connect to the internet . Set your first card the interNIC to dynamic host unless you have a static IP. The second card should be set to static IP address "192.168.0.1" (server/clientNIC). when you connect into the hub you should connect the host/server PC into 1 of the regular "downstream" ports. If you connect into the upstream port you will need acrossover cable.
Client PC's connect these into the other ports on the hub. These should be set to static IP addresses ranging from 192.168.0.2 and on up for each PC. The default gateway and DNS server should be set to, the address of your host PC (192.168.0.1). Also make sure you put in that PC's name in the host field on the each PC, leave all the rest blank.
By doing this you should be able to network into the PC's and share your internet conection from your host. Another thing to check also is to make sure that file and print sharing are enabled on you win98 machines in order for the win2000 machine to see them.