Unreal tournment GOTY for sure its easy to run and PM me i can get you it. Starcraft is pretty fun to but diablo2 IMO died the new patches turned the hole game around thats why i quit.
Diablo 2/Diablo 2 Lord of Destruction
Warcraft 3 Battle Chest
Unreal Tournament Game of the Year Edition (If you can find it)
Command and Conquer Series
Does it have to be multiplayer? Hitman 2: Silent Assassin has some nice graphics and great gameplay, and I used to max out the settings on my old GeForce 4 Ti 4600.
I would go with Deus Ex. It should run smooth as silk on your pc, and it has just incredible gameplay,great story,and multiple endings. I wish they would still make games like this.
Also as someone has already mentioned. Unreal Tornament Game of the Year is a great game that should also run smooth.