So, the x1600 worked properly in another computer?
The x1600 gets 12v voltage from the AGP slot on the MB. That voltage is regulated by the MB voltage regulators. Could be driver related. If you can get to the desktop, perhaps go to control panel and delete the ATI graphics driver for the 9800. I linked the proper driver for the x1600. Also, try booting the system into Safe mode and installing the x1600 driver.