Need some help w/Old Dell graphics ... (maybe BIOS tweaking needed ?)

I started out on Macs and have never really gotten into the details of BIOS, boot sequence, etc. But now I'm trying to boot a Dell Optiplex 760 with a PCIe graphics card, and the Setup doesn't seem to allow defaulting to the on-board video when a PCIe video card is installed. If your graphics card is PCI, you evidently have that choice, but if a PCIe card is installed, that is automatically chosen as the default video. Well, I want to use the PCIe card for GPU computing (which I'm actually doing now) without having it slow down the GUI. Right now it's causing horrible delays in response to user input, because it's trying to run a CUDA program while constantly updating the user interface. Is this the kind of thing that someone has already written a BIOS mod for, and if so, how could I find it ?

Alternatively, is such a BIOS tweak known for the Optiplex 789 ? It appears to have the same issue:

Thanks for any answers. Keep in mind I'm not really familiar with the Windows/Intel world, so explanations may need to be a little more verbose than usual.