Sims 2 is old-ish, but still a good game.
Roller Coaster Tycoon 3, ditto with above.
You could also go for the old classics, (DOOM, Daggerfall, etc.)
Also, most emulators besides GameCube and Wii one(s) would run okay on your computer.
Should this really go in the GameCube, Xbox, and PS2 section?