Installing a basic clusters requires at least 2 machines with network connected. Either using a crosscable between the two network cards or a switch or hub. Off course the faster your networkcards the easier you will get better performance for your cluster. These days Fast Ethernet is standard, putting multiple ports in a machine isn`t that difficult, but make sure to connect them through other physical networks in order to gain the speed you want. Gigabit ethernet is getting cheaper any day now but I suggest that you don`t rush to the shop spending your money before you have actually tested your setup with multiple 100Mbit cards and noticed that you really do need the extra network capacity.