More thoughts...

Anyone know if the PM6400 and PM6500 riser cards are interchangeable?

My thought is to trace out the C600 riser card connections. If they match (as far as the two PCI slots go) the PM6500 riser card connections (have a 6500 on hand, but no 6400), then I might risk trying the C600 riser card in the PM6500. Testing the functionality of its slots will tell me whether the needed resources actually go to the riser connector or must be hunted down near the chip where they originate.