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16 Xbox hooked up in System Link (Halo, Rainbow Six 3, Ghost Recon ...)
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Bawls
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PCs (this has already been covered in other posts)
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Lot's of tables and chairs for board game, collectable card games, role playing games, ... Maybe even for chess, who knows!
Also consider making your Gaming Center a store for theses geeks. Why not sell video games, collectable card games, figurines, comic books,
... You could even sell and repair PC hardware, since you will probably need a technical person there at all times anyways.
Also, consider allowing people to bring their own PCs, specially for big events.