probably ur clg will be havin some switch or somethin. just unplug all other pins from it temporarily nd connect only the required ones.
in my clg when there was a gamin competition, i did the same. There was a large shelf like thing, which was a huge switch and did the connecting stuffs and bingo.
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and the best game combination will be
* one team based game like CS Source (1.6 is gr8 but too outdated)
* one fps like UT04 or UT3 is systems can take it.
* one race game like MW or carbon.
* maybe a strategy like AOE3, AOM etc but itll take a whole day
and leave some systems w/o networkin for some timepass or practicing for the players or spectators.
and it wud be gr8 if u can connect an wifi router to the n/w so that audience can spectate the game on the go nywhere in compoud, nd do remember to set some player password or they ll enter the event w/o authorization nd spoil the match.