Be careful it all depends on how lucky u get as which card u get and how well it overclocks
taken from bjorn 3d
As my test system specifications showed earlier, my system is loaded with components. This might be a reason why I simply couldn’t overclock the BFG 6800U OC any further than 427/1150. BFG did tell me that they have gotten boards to 450MHz, but their test systems are not as loaded as mine. I am also working on a review of a 6800 GT from another vendor, and this card is overclocking well in my system. Unfortunately though, BFG's 6800U OC just didn't overclock more than what BFG already has done.
http://www.bjorn3d.com/read.php?cID=655&pageID=833