Heres my advise...
First of all I have the same goal really, a quiet computer. Right now my computer is:
Antec case
1 Panaflo la1 in (7volts)
2 Panaflo la1s out (7volts)
1 Panaflo la1 (12volt) in the powersupply (= Antec PP412X)
1 Panaflo la1 (12v) on the heatsink (= Alpha pal8045)
mobo= no fan
videocard = no fan
harddrive = 80gig seagate cudaIV (24dba) in a home made thign to quiet it which works kinda well
So total it is bout 17+17+17+17+21+21+~21= 27.6dba
I'd say the loudest thing right now is the powersupply... after that its jsut the case fans moving air..
If i was gonna improve upon this, I'd replace the powersupply fan and the 8045 fan wiht 15dba Papst fans.
I'd leave teh side of my case off and just have 1 fan 7v fan blowing out behind the cpu
I'd maybe replace the powersupply with a pcpower&cooling (better quialty = better at taking high temps)
Few things
Don't buy a "quiet" powersupply, buy a really high quality one... and replace the fan with a quiet one. Enermax = not really high quialty I'd say pcpower/cooling or antecpp412x (havn't seen any othe rantec pwoersupplys)
www.2cooltek.com has a heatsink to replace a mobo's hs/fan setup.. it's made by zalman and its just a really tall heatsink - well worth it. mobofans = stuidly loud
if you don't have a harddrive get a cudaIV... nothing comes close to their sound 40gigs=20dba 80gigs=24dba
pal8045
try to get a videocard you can put a big passive heatsink on.. who cares if it blocks the next pci slot