There exits Sonnet specific nvramrc patches for Sonnet card PCI-PCI Bridge problems that you can download.ISTR (perhaps trag can elucidate) that there is some issue with cards with PCI-PCI bridges and the 6400/6500. That said, my TAM had a Sonnet combo USB-FW card in it and the TAM is basically a 6500, so I can recommend one of those.
Paging @trag ...