The Apple 30" Cinema HD display is 2560 x 1600 and the dual link 6800 Ultra can support two of those at the same time.
I did a quick check of the nvidia web site and could not find specs for that card.
Don't know how to answer that other question except to say that Motion 2 uses it in 16/32 bit float, so I would say that it does accelerate in 16 bit.