The BIOS manual, page 4-8, for the SUPER P6SWA it states:
Default Primary Video
Choose External when using a PCI video card and Internal when using the onboard graphics. Although External is the default, if no video card is detected the onboard graphics will automatically be enabled.
Remembering way back when working on a motherboard similar to this one I've encountered IRQ conflicts that would stop a discrete PCI video card from being detected. The solution was to move the video card to another PCI slot or to play around with manual IRQ assignment.