Ok, what you need is an Xbox Wireless Adapter
Here's the description:
Add your Xbox video game console to your high-speed wireless network with the Xbox Wireless Adapter. Play games on any TV in your home
without wires and with the assurance of the leading wireless security.
You can buy it today
Here for $99.00, I know it says $139.00 but when you click on the "Buy It" link, the rates on circuit city and similar places is $99.00.
Or you can shop arround for something cheaper... just check the speed transfer.
Hope it helps. Good Luck!