What I did to quieten my PC - it's not silent but much better than it was (it used to sound like a Harrier Jump Jet with takeoff problems, mostly from the Xilence PSU - misnamed in my opinion, they should rebrand themselves as Xucking_noisy_xastards; replaced it with a Be Quiet one which is much better) was to reduce the disk drives to a minimum by offloading them into a PC in another room connected via mains networking, swapped the CPU fan for a very quiet PWM driven fan so it would only respond to CPU heat, disconnected all other unnecessary fans and introduced a fan speed controller for the remaining fans (yeah, it's an Antec 1200 with 6 case fans and 2 CPU fans, plus the gfx and PSU fans which I wasn't willing to fanny about with).
The other thing I did was to measure the *actual* power consumption with one of those energy monitors and picked the Be Quiet PSU rating based on that. Peak consumption was 130W with a stress tester running so picked the new PSU accordingly.