don't listen to the people above me. there not quite right.
I, personally, have hosted 5 Xbox and 2 PC lans, and have organized a weekly thing at my school and am currently organizing our Class Night Lan, which will be the biggest weve ever had, 2x 16 player lans, playing Unreal Championship, Halo 2, Battlefront 2, etc...
but enough of my resume.
youll need a few things.
1) A network hub or switch, or a DSL router if your in a jam
2) strait through, CAT-5 cables that can reach from the hub/router/switch to the X-boxs. you could have the hub at an intermediat distance and have two equal length cables going to the boxs, or have it next to a box with a short connector cable and a long one that can reach to the other one.
the cables can be pretty expensive if you buy them pre made. $30 or so if you want a 100 footer or something. I make my own, pretty cheep if you have a real of the stuff and the $50 crimping tool, but I assume that's out of the question for you?
so in the end you will have it go
Xbox -> Cable -> Hub/switch/router -> Cable -> Xbox