Alien Swarm
-- It's the most fun when it gets hectic, so always make sure to keep your difficulty high. Even if you fail the level, it doesn't really matter. If you go through all the levels without a hitch it's not gonna be as fun.
S.W.A.T. 4
-- This is a golden oldie IMO. As the name implies, you're SWAT, and you gotta take down some baddies, with hostages and possibly bombs etc. Trick is you gotta try and take them alive, though. And obviously you got Tazers, pepper spray, flashbangs, gas grenades, all that fun stuff.
Neverwinter Nights 1 & 2
-- These are in the higher end of the CoOp challenge. You can choose to play community made custom adventures or the game's own campaigns. There's a twist though. You can also BE A DUNGEONMASTER and GENERATE THE LEVEL IN REALTIME AS YOUR FRIENDS PLAY IT. That is by far the most crazy awesome AD&D EVER! It should be said that NWN 2 obviously has more features, but it generally considered inferiour in storyline and has it's share of technical bugs with framelag etc.
Nazi Zombies
-- Whether it be with World at War or the upcoming Black Ops, this stuff is solid gold. Not sure if Black Ops will run on your rig, though.
Hidden & Dangerous 2
-- This is one of my favorite franchises. It's a shooter putting you in the shoes of a four man SAS team during events of World War II. In CoOp, each player can control one of the four guys, and in single you can control all of them yourself, with the help of AI. It imposes (fairly realistic) limits on carrying capacity, and many of the individual missions require you to bring gear from the previous ones or find in enemy stockpiles when needed.
