So you are saying that when the Radeon 9200 (PCI) is installed in the QuickSilver, the Radeon 7500 (AGP) works as it should. But when you remove the 9200, the QuickSilver boots to Single User (command line interface) mode?
What happens when you hold down the Shift key at start up?
Have you tried running the ATI installer to uninstall the Radeon 9200 drivers? I don't know why it would really matter, though, as the software ought to be smart enough to figure out what drivers to load and what drivers to not load...