Just re-reading the thread and saw this afresh:
It does detect my PCI mounted Nvidia FX5200 Graphics card but it still chooses the
onboard Via graphics drivers.
Can you get the system to come up on a monitor attached to the onboard video port? If so, set up that way, then get in PCC and select the Nvidia from "Browse and Configure Hardware", swap the monitor over to the PCI card, and reboot.
Might do the trick for ya?