I have used NVIDIA 7950GX2 with this board but it worked only with BIOS 3044 or older while my new ATI 5850 works with any BIOS version including the latest one. My conclusion is that the board seems to be picky when NVIDIA cards are involved. Try flashing your BIOS to 3044 and see if your card works. If it doesn't, then try forcing PCI-E 1.1 compatibility mode for the slot. If your power supply is good enough (as in minimum 550W of real power) that's all you can do before replacing something.
Do not mix two different brand cards in one system.
BIOS should not have an effect once the OS has been started unless the mainboard has on-board graphics that are not turned of in the BIOS.
A modern OS needs no bios. The BIOS is only needed at boot stage and yes, if the BIOS detects a wrong video adapter, sure you won't be able to install the correct one.
An if changing video driver, first uninstall the existing driver so that the OS fall's back to its generic VGA driver.
if a computer starts, the first thing that acts is the Video BIOS or you would not see anything on the screen.
The mainboard BIOS is there to detect all installed hardware devices, well, things like IDE/SATA devices, keyboard and USB controller and if an audio controller is also on-board, audio detection.
On windows side: check that windows detects the correct card and if windows can't find the driver then download the driver from the video chipset manufacturer, in this case Nvidia.