Ahem. DOOOOOOOOOOOOOoooooooooooooom!
(Getting Doom '95 will make it really easy to set up)
Also, if you can find someone to play with regularly: Baldur's Gate
Freespace 1 should run fine too - this makes a great game for some co-op. I concur about Incubation; although for a big game (3+ players) you really need something else to do while the other person is taking their turn!
Reading slightly more: If you have someone who'll play whenever you want to, then Diablo is great. Otherwise, you'll end up out-levelling them to the extent that it's unplayable in co-op
Happened to me...