But if you want silence heaven, even at load, I suggest the Powercolor 6850 SCS3 with a 800 rpm Slipstream fan zip-tied.
Link to product at Newegg: http://www.newegg.com/Product/Product.a ... cs3%206850
I have this setup in a Solo case, and with the fan set to ~600 rpm, maximum GPU temperatures I get in games is about 66C.
When running Furmark however, I have to turn up the fan to full 800 rpm, to keep it reasonably cool.
600 rpm is almost silent, 800 rpm is still quiet. You should be warned that this combination takes a lot of space
Let me reiterate this comment. I think that the passive 6850 from PowerColor is a nice option, but as Tzupy said, it's large. Width alone is over 2.5 inches, which would be problematic depending on your setup. Toss a Slip Stream on top and you've added another inch.
That said, it's pretty much the top dog for passive cards at the moment. The passive offerings for the Radeon 6770 and 6750 cards are much smaller, and can do general gaming (though the 6850 definitely has better performance), but are significantly smaller in the case. IIRC, the 6750s I installed (http://www.newegg.com/Product/Product.a ... 6814131449
) are 40% smaller (by height). That can make a huge difference in your case.
Just remember those items when you're looking at the card. It caused me to take a step back and re-evaluate what was really needed for the box I was using.