I am eyeing this one, http://www.x-arcade.com/htm/landingpage-fight.shtml
because of all the fighting games coming for the XBox and my countless ROMS. That one is expenisve but it seems to be the only one for the XBox or you can check EBay for those limited editon Soul Calibur sticks that came out a while back and I hear there is one in the pipeline for the King of Fighters series.
Compatable with all systems and PC. I will be getting the 2-Player version in the future.