Thanks Lonni, acpi=off did the trick. I can see the second VGA card on the PCIe bus doing 'lspci -t', although I think it is not located in the right node. Of course I lost ACPI control.
FYI, this is kubuntu kernel 2.6.15-23-amd64-k8.
I am now happily running with 3 monitors, 4 soon