A device called a repeater can increase the signal strength by rebroadcasting what the router is sending. Another alternative is to use a device called a bridge. It lets you connect the xbox (or computer) with an ethernet cable to the bridge. The bridge then works wirelessly to connect to your router.
I have better luck with bridges to do what you are talking about. They tend to have better antenna and transmit power than most wirless adapters that are built in to computers, xboxes, ps3s etc.