I finally tried a Voodoo4 PCI in my PowerMac and can confirm that the MorphOS driver doesn't work properly. I remember that we adjusted the PCI setup for Pegasos systems on early quark init for Voodoo4/5 cards which is not possible for PowerMac. The driver will start but the screens will be rendered with messed up colours. It's not sure yet if this can be fixed for a future release.
Voodoo3 should work OK, though. You can use a standard PC BIOS card if you only want to use it with MorphOS.