OK, I'll admit right away that I probably paid too much for it ($50 CDN) but I wanted to see if I could get the best possible effect.
I purchased a SilenX Luxurae Hard Drive Silencing Solution from Bigfoot Computers
It's a pretty simple affair. Basically two enclosure halves made of steel, a bunch of screws, two heatsinks, two foam pads and two strips of thermal tape.
Installation was so dead simple, I immediately realized I could have probably done all of this myself with some off-the-rack parts, but the time and effort to track them down almost (almost...) justified the cost.
You secure the self-adhesive foam pads to the top and bottom of the enclosure. Then apply the thermal tape to the heat sinks. Screw the heat sinks to the sides of the HDD. Rest the HDD with attached heat sinks inside the enclosure, making sure you have decent clearances for air flow. Put the top on the enclosure and screw on the sides with the countersunk bolts. The entire enclosure then slides effortlessly into a 5.25" bay. Plug everything in, and presto!
With a Seagate Barracuda IV (edit: it's actually a V, not a IV - my error) SATA 200G HDD inside the enclosure and inside the Fong Kai FK-333 case, it was silent for all but the most intensive read/write cycles, and even then, it was tiny, tiny whispers of noise. Most of the time, I had to look to see if the light was flashing to see if the drive was running.
Without the enclosure, the Seagate was fairly quiet, but grinded a bit on seeks and writes.
If I had to do it over, I'd probably go with a Sunbeam elastic suspension system because it's probably just as quiet and a lot cheaper ($16 vs $50) but I'm very happy with the HDD noise (or lack thereof) right now.