If you don't have the AGP miniport driver installed yet, the nForce UDP (found in NVIDIA's driver download section) contains pretty much everything you need for the motherboard portion (under platform drivers).
first to get rid of the old display drivers. When uninstalling, make sure to ONLY uninstall the display driver and not the drivers for the GART, audio, SMBus, etc.
Usually your BIOS will automatically disable the integrated graphics once an AGP card is detected. If this is not the case, allow the OS to automatically detect the integrated graphics, then disable it through the Device Manager of the System Properties (Windows NT/2K/XP) or the System Properties itself (Windows 95/98/ME). From there you can install the new drivers.