Hardware requirements

Installing a basic cluster requires at least 2 network connected machines, either using a cross-cable between the two network cards or using a switch or hub (a switch is much better than a hub though and only costs a few bucks more). Of course the faster your network-cards the easier you will get better performance for your cluster.

These days Fast Ethernet (100 Mbps) 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 every 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. Next to putting a Gigabit card you might also want to try bonding different 100Mbit cards together. An even cheaper alternative can be found in Firewire, as discussed in this paper