If you wanted to go the wired route, you could probably get away with just a simple cheap hub. You can't just split network cables like you can phone and cable TV cables.
If you wanted to go the wireless route, you'd need to get a wireless router. Preferably one that advertises compatibility with Xbox Live.
It's up to you which way you want to go.
We just got broadband internet and since Halo 3 is almost here, I'm finally thinking about getting Live. I was wondering, since it costs alot less for the cable modems, if is cheaper and easier than the Xbox 360 Wireless Reciever. And if I did that, would I have to buy a cable splitter, so I could hook up the cable modem. Thanks alot
treemen

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic