Just an update.

I changed the video card (GeForce 8400 GS) to a Radeon x1650 Pro. Problem is still there. QE/CI, resolution, everything works. But when I hit shut down, then turn on the PC and let OS X boot, it gets stuck in a blue screen right before going to the desktop. If I boot Windows first, then reboot and let OS X boot, OS X boots perfectly. It seems the problem is with the shutdown. On a side note, if I remove the video card drivers, OS X always boots correctly, even if I shut down. So the problem is related to shutting down and having the card activated (meaning QE/CI/Resolution).

Have tried with 3 different video cards. For now I give up. If someone has a clue as to why this happens (I have no idea) please let us know. I have searched the net and it seems no one else has this problem.