If you're looking to network them cheaply, pick up a couple of Network cards and a cable to interconnect the two. if you need to connect more than two then you'll need a hub or switch or router to connect to. Be careful with the cables becuase two wires are twisted and connected differently for not using an ethernet hub/switch.. and then there are the normal ones. The sales people should be able to help with whichever route you'ld like to take. Network cards run $5-$40 depending on how much you'ld like to spend. Ethernet 10/100 are the commonly used ones now. The cable should be at least 3 ft long otherwise you could burn out the network interface cards. There are a lot of sites for how to set up a home network, and plenty of books andif you buy a complete kit it'll come with all the instructions you'll need as well. I haven't seen a usb network setup but I know it can be done.
Good Luck!