You would not notcie any difference in your xbox speed. A standard DSL speed is about 320kbs (your actual speed is slower), while wireless 802.11b is 11Mbs!
Besides, you would have a cable from the modem to the router, then another cable from the router to the xbox. There would be nothing wireless about the xbox connection. Just understand that, even at the lowest wireless signal strength, your wireless connection will be faster than your internet connection could ever be.