Win XP 32bit service Pack 3, single-core AMD Duron 3000, my computer is 13 years old. Why I use it? because it works and refuses to quit. It would feel like throwing away an old dog that lives with you for years just to replace it with a puppy. And yeah, in run into slowdowns on bigger X-Piratez levels, especially when multiple floors (>4) are used.
Apart from that my PC does everything I want it to: Play Doom (Zandronum), Play OpenXCom, watch youtube videos and DVD. I even have 2 monitors and there is no to little lag. and the stabilty: Ecellent.