The scream is from the Video card, and means that it is not receiving power from the AUX connector (or perhaps from the AGP slot, though I don't know this for sure). I think you may have a bad Ultra, since you have been through two motherboards already. I would guess that there is something wrong with the AGP connector of the card, which caused the artifacting when it loosened. I would make sure that the card is firmly in place, that the screw in back is tight, and that you can lock the little AGP lever.
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