just pick up a "cable/dsl" router, and a 5 port switch... please, for the love of collisions, dont get a hub...
also, a router is not just a switch with a firewall, its worth the extra 15 dollars for the DHCP server that is built in, it will make the xboxing alot easier on you, and the PC as well. and the firewall is not really a feature, its more along the lines of you dont have a choice in the matter, cause with out NAT, you have nothing but a switch...
CODE
internet-----modem----router-----switch-----------------
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PC xbox xbox xbox xbox