If this is the machine I believe it to be, you will need a network adapter PCI (not PCI-e) expansion card that contains an Ethernet port, or a wireless port. You should be able to select a card of either type for a reasonable price.
EDIT: Examples of the cards follow. You do not need to choose one of these as many vendors make cards of equivalent capability.
Ethernet card: http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&N=-1&isNodeId=1&Description=pci+network+card
Wireless Card: http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&N=-1&isNodeId=1&Description=pci+wireless+network+card