Disable Default VGA To Enable Another Graphic Card

I have a somewhat older Dell that is more than capable of running Windows 7 (it also runs Server 2008 R2 quite nicely). The problem is that the onboard ATI graphics only has 16MB of RAM and is only recognized as Standard VGA Adapter. This is really unjust, so I obtained an NVidia FX 5200 for next to nothing and placed it in a free PCI slot.

Much to my surprise, the BIOS has no settings for disabling the onboard video. It also has no settings for instructing it to look to the PCI first. As a result, the onboard video is always active during POST.

I disabled the onboard video from Device Manager, shut down, and installed the new NVidia PCI card. I rebooted with the monitor attached to the PCI card. Nothing. I switched the monitor to the onboard video and sure enough, there it was. The NVidia had also been detected and was showing up as a Standard VGA Adapter with Code 10 -- cannot start. Not a problem, I thought. I had yet to install the NVidia driver, and the onboard video had already claimed the default VGA during POST, so this was not surprising.

I installed the recommended 96.85 Forceware drivers for the NVidia FX 5200. These are the Vista 64bit drivers. Windows accepted it without problem and I swapped the monitor back to the NVidia during the reboot. What I was expecting is that the display would be blank during boot because the onboard video is active. I assumed Windows would switch to the NVidia when it started because I had installed the driver and instructed it to disable the onboard video.

I was wrong. Blank display. I switched back to the onboard video and my display returned. The Nvidia card was recognized in Device Manager and it was showing "This device is working properly". The onboard video still had the disabled icon next to it. Yet I cannot choose the NVidia when I go to Screen Resolution and other screen preferences. It is using a blank video adapter -- looks like VgaSave.

Have I gone wrong somewhere? Or is there a more appropriate driver I should be using? Thoughts and suggestions would be helpful. Keep in mind the onboard video cannot be disabled in BIOS, nor can the BIOS be instructed to look to the PCI first for video. There are no BIOS updates available. There is no AGP slot, only two PCI slots. This is what I have to work with.