I have absolutely no experience with aftermarket GPU cooling, so I hope someone can point me in the right direction here.
My HTPC rig currently runs an Asus HD 6670, which started out as too loud for the job even at minimum 40% fan speed. It was a leftover from a dinosaur rig I parted out to other builds. Well, today the fan started wobbling and making all sorts of noise, and I found a screw had worked loose on the heatsink. I took the entire assembly apart and reattached it with new thermal paste, which seems to all be fine for now.
However, this made me wonder about the possibility of using a newer, quieter cooler- maybe even a passive heatsink. I don't know if such units are universal in fit, or where to even begin to start evaluating their comparability. The passively cooled 6670 cards are way overpriced to buy new, and the common passively cooled models (5450, ect) are probably not going to handle hardware acceleration of HD content as well, if at all. Even if it were to cost nearly the whole value of the card, an effective passive heatsink would seem to be the most logical path...
if i were you i would try to solve this via DIY method first before buying another after market cooler. for example attaching 80mm case fan to the heatsink. personally did this to my passive cooled 4550 and 9600 gso. and yes it was dead silent
There are not that many passive coolers for graphic card and they can be large . Therefore, you need look at your case to determine if it possible to use a passive cooler.
For example, I have a passively cooled nVidia GT 9600 GT and I know it runs hotter than the Radeon HD 6670. It is passively cooled by an Arctic Accelero S2. I did not attach it myself, it was sold that way. I have not seen the card in about 2 years because it is naturally in a computer case. The cooler takes up the adjacent PCI-e slot. Many cards take up the adjacent slot anyway so that is usually not a big deal unless you have a micro ATX motherboard. The major concern is height. The Accelero extends about 1" - 1.5" above the card so if you do not have that much clearance, then you cannot use the cooler.
Unfortunately the ARCTIC Accelero S2 has been discontinued. This is what it looks like though.
However, the Accelero S1 is still being sold for $40. Not exactly a cheap solution. The difference is that the S1 is longer (240mm vs about 190mm); that's just under 9.5" long; not too bad. It also sports 4 heatpipes rather than just two.