The easiest thing for me, since I had two bedrooms with wireless computewrs and game consoles and a living room with a media center computer was to buy the Zyxel Wireless P330 router at Compusa for $49. It is configured to either be a router, a bridge, or a gateway. I use it in the bedrooms as a bridge. It has 4 ports. It connects to my main wireless router in the living room. I have 2 computers and an xbox in one bedroom connected to it and a desktop connected in the other bedroom. I do not have to worry about my main computer being on all the time so I can use the other computer or the console.Also, this means I have two backup routers in case the main one breaks down. And $49 for a wireless/gaming bridge is cheap. It works well and has good range.
Compusa/Zyxelusually the stores have them in stock. ICS is free but it can be limiting when you are using the main computer and want to use the console as well.
In answer to your original question, what is the subnet for the xbox360 set at?