First, that kicks ass that you can play Halo with everybody in the dorm without having to pony up the money for xbox live. Second, I tried a RJ-45 splitter I found at your locel radio shack but I couldn't get the thing to work. Why can't you just get a router? I don't know about configuring the xbox to it but usually they're PNP anyway. Here's one on newegg for $32.
Router