i just got a 6800 a few days ago.... i unlocked it to 16/6 and it overclocks quite well too I used rivatuner. there is a thread on this board that tells you how to do it
It's impossible to unlock the extra 4 pipes and 1 shader if you have a 6800nu PCI-E. I have one, but I'm able to OC it from 350/600 to 457/842 which kind of makes up for some of the pipes.