My initial post for this thread didn't go through for some reason, so I'll add one now. I had been experiencing hard lockups while playing Civ4 which would force the video card into standby mode. My problem ended up being caused by a slowly failing AGP port. I replaced my mainboard with a new working one and my Civ 4 problems are gone now. I can see that the game has some minor bugs in normal play but I haven't had a crash since my defective hardware was replaced.