The performance of the vf900 is dependent on how your mobo is oriented and what kind of airflow you have around the card. If you have a normal ATX case with the mobo upright and not much airflow in the card area then the vf900 won't do well (same with the accelero). If you have a fan over the cards then you have no trouble, or if the mobo is upside-down that allows the heat to rise away from the card. So they do work fine on X1900 series cards, but just dependent on case and airflow layout.