Redjake, those are the standard things to look at. However, I tried the first, a similar design for my PSU (hence the "silicone barrier" I talked about). The pax mate sound absorbers would probably only filter the higher sound frequencies - it's too too light to stop vibration. Finally, the harddisk isn't the problem.
Vibration of computer cases can be stopped by applying enough weight to them, making them too heavy to vibrate. That's why I thought about rubber or floor tiles - they're heavy. I did some research on the web, and that would seem to be the best way. A product that would come close would be <a href="http://www.cluboverclocker.com/reviews/other/dynamat/">Dynamat</a>. That's rather expensive though...
The reason I started out in Tilted Electronics was that there must be someone out there that has the same problems in a non-computer environment. car engines + hood vibration, or speakers + container vibration. If there's a cheap and effective way to stop that, it might be applied to my computer.