PSU - I can recommend the Corsair HX520 PSU.
SilentPCReview gave them a great review. I have 2, not silent, but very quiet. My brother has a passive PSU, quiet, runs hot and needs case fans, which ends up being noisier.
CPU - You should be able to turn the CPU fan off. Mine only turns on at 50C
HDD - I use Samsung 500GB in my HTPC that are pretty quiet. Just got 2 WD 500GB Green Power drives for my WHS that are even quieter.
Grahics - passive card
Water cooling - put the money into the other components, one more thing to go wrong.
The key to running a cool and quiet system is good air flow. I have an LC17 and sealed off various vent holes so that the air flows with best cooling results. Used a Speedfan plugin for MCE/VMC to monitor temps using various configurations.
NOTE: The WD Elements External enclosures use GP drives and can be bought very cheaply atm (I paid $150ea for 500GB).