I used to have a great coop setup with 2 - 4 PC's (though granted most of them were well ****) I know quite alot of good FPS COOP games:
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Operation Flashpoint - This is old but I prefer the gameplay to Arma and Arma2 and much more suited to COOP in my opinion. It also has better mods.
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Sven Coop - Half-Life 1 mod, still being developed and has amazingly fun coop
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Synergy - Half-Life 2 mod, very fun.. has the Half-life 2 SP Campaign available in Coop which is fantastic.
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Serious Sam: First & Second encounters - This is a brilliant coop game, alot of fun and stupidity to be had. Theres no real challenge it's just blasting monsters, Theres a new High Definition version of both games just come out on steam that has new COOP mods too aparently, might be worth a look if like you say you have good graphics to go. Serious Sam II is wank though.
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Doomsday Engine - Gives Doom, Doom II, Doom Final, Heretic, Hexen complete 3d graphics make overs + improved net code for smoother multiplayer/COOP games. I highly recommend this one.
theres more, those were my favorites though.