PCI express card not seen on system with onboard video

I recently bought an ATI Radeon X1300 Pro PCI-e card and installed it in a Dell E520. The Dell has an onboard video card that works, but this is supposed to be automatically overridden when the PCI-e card is inserted. The new card powers up but there is no video coming from it and the driver installation app doesn’t see the card at all. I am using Windows Vista, anyone have anything I can try? I have scoured the BIOS settings to no avail.