http://www.engadget.com/2007/01/09/riteks-16gb-and-32gb-ssds-starting-at-a-low-low-169/The point of diminishing returns come when you swap a $100 300GB hd for a $600 32GB SSD drive, etc. The last 10% of noise is simply very expensive to get rid of, and will almost always affect performance.
Give them a year and SSDs will be a very attractive proposition
I do agree that getting a very quiet or silent PC with decent performance isn't easy and can be pretty expensive.
I built a PC a couple of years ago and was going for "quiet". It never quite got quiet enough. Building my HTPC with the mCubed case i was going for silent. You can just hear the spinning hard drive (well i can), so it isn't completly silent, but it is very acceptable