I wifi tether to my laptop and run a Ethernet cable from it to the Xbox. I run modern warfare 2 multiplayer perfect off my 3g. On a average uses about 6 to 8mb per hour. I don't download nothing off there though.
I have windows 7 and just bridge my wifi connection and my local LAN for it to work. On the Xbox itself if u test connection and come up with a ping error, that is normal. But just sign in anyway and it will work.
Hope this helps. I don't know if u can wifi tether to the Xbox itself as i do not have a wifi adapter.
Sent from your mom's phone.