What a coincidence. I just installed the WhisperRock II on a Duron 1.6 in a computer I built for my niece for Christmas.
The WhisperRock works fine, though I didn't really take the time to monitor the CPU temperature. I believe it was reading somewhere around 38 C in the BIOS, which is no indication of normal performance. At any rate, it will keep your Duron running adequately cool, and the fan happens to be extremely quiet. If you're standing a few feet away from the computer, you can't tell it's on.
There are a couple caveats, though, which made installation a little bit of a nail-biter. First, the mounting clip only has ONE notch on both sides. I'd much prefer to have three, but as the cooler isn't very heavy, I guess I can let it slide. Second, the screwdriver slot in the bracket is very small, so you have to either use a skinny flat-head screwdriver, or risk stabbing your motherboard (never a good option).
All in all, I'd say it's a good cooler, and so quiet. I'm using one in the next non-XPC computer I build. Great for the price.