The recent launch of a passively-cooled Radeon HD4670 from HIS sparked my interest. HIS's latest offering is based on the same iSilence 4 heatsink that their passively cooled HD4650 uses. The company was able to keep the card cool enough without underclocking, which is a common resort when trying to passively cool. The card retains its reference clock speed of 750MHz and 873MHz for the GPU and memory, respectively.
You will have to pay a steeper price for the card which is not surprising, but still makes for the fastest Radeon in this segment that is passively cooled from factory. For extra performance (and added cost) there are a few different fanless GeForce 9600GT cards available and a Gigabyte Radeon 4850 card in what appear to be the fastest graphics cards you can get today that are passively cooled without going the water-cooling route. If complete silence is what you want, the extra price may be worth it.