I finally got so fed up with the noise coming out of my computer I've started to do something about it. After some careful listening and poking around, I figured out that the GeForce4 Ti 4400 graphics card was the worst culprit. The small fan on the card made a nasty loud high pitched whine.
I ordered the Zalman ZM80A-HP from Quiet PC on Sunday night and received it Tuesday morning. It's a massive (480 gram) passive heatsink that completely replaces the original fan. The installation isn't difficult but taking it slow and carefully, it took me about an hour.
The results are fantastic. It works a treat and that's one nasty fan eliminated! Unfortunately, there's still five other fans and now the computer just has that low level whirrr of an air conditioner! I recently found the Recommended section of Silent PC Review website and I'm mulling over my next move.