If you are connecting to the ISP on ethernet, you need two ethernet cards on your gateway system. You may either install two single-port NIC cards in PCI slots, or select a dual-port NIC card. If you have a NIC slot built-in on your motherboard, you may go for a USB NIC device, instead of going for a second PCI slot. Choice is yours.
You need a NIC card one each on all your workstations (may be PCI, ISA, USB, wireless or built-int).
You need 10/100 network hub to connect all your workstations to the gateway system. Use a Cat5 cable for all connectivity reasons. If you are using uplink port on the hub, use a crossover cable and leave the adjacent normal port empty. Otherwise, if are using a normal port, use a straight-through network cable. If you have all similar cables avoid using the 'uplink' port on the network hub.