Hello,
You will not necessarily need to purchase a cable modem, your cable providers may provide you with one that you can "rent". Other than that you need 2 NIC's, which only cost about $20 a piece for 10/100 cards. Also if you want to share your cable connection looks into purchasing a cable/dsl router. I heard linksys makes a good one (
www.linksys.com). You will also need some CAT 5 patch cables, but those shouldn't be too expensive either. Hope this helps.