The Presto Gigabit Pro ExpressCard/34 is fully compatible with Snow Leopard, both 32- and 64-bit modes. In 64-bit mode, however, a new setting is required. Delete the old setting, restart, and manually create a new setting for this card.
Yes, the Presto Gigabit Pro and Presto Gigabit Server cards (PCI, PCI-X, PCIe and ExpressCard/34) support Jumbo Packet transfers under both Mac and Windows.