I just bought an Asus hd6950 DirectCUII 2GB and i can tell you that you have nothing to worry about with temps and noise. I have mine unlocked and overclocked and the fan doesn't go above 20% when loaded.
It is an awesome card if you have the room for it.
The Asus DirectCU and MSI Twin Frozr's have some of the best cooling on the market.Shouldn't need to worry about temps with those cards.However the Asus card is a triple slot card.So if your going to crossfire make sure you have the space.
As for unlocking the cards I have heard that Sapphire makes good cards for that but that was awhile ago.You'll have to research it and check it out.