Yeah- silent performance with either Macs or PCs is really not the great mystery people sometimes make it out to be. It's really not a Mac vs. PC issue- it's a cheap crap vs. quality issue.
A a video card with a heatsink rather than a fan makes (all together now class) no noise. Period.
These days most motherboards have heatpipes and sinks not fans. No noise.
Watercooling hardware won't magically start making noise- unless it springs a leak, then I guess you'd hear a sizzling noise.
A decent PSU won't make much noise. Decent case fans, properly mounted and rated for low noise won't make more noise unless they're damaged or something. Hard drives can easily be made virtually silent by proper shock-mounting. A decent third party CPU cooler can easily be silent. Companies like newegg post all the db level noise stats on hardware like that.
It's often been my experience that people that build PCs with fairly decent hardware then complain they're too loud, or suddenly 'got loud' -presumably by osmosis- didn't know to turn on hardware fan control in the BIOS,or worse, never hooked their fans to the proper control headers to begin with.
Most of my own PCs are dead silent- you don't even have to spend that much money to do it. Some $7 120mm case fans to replace a stock, a decent CPU cooler (maybe $50), passive video card cooling, shock mounts for anything that could rattle ($5 or so) a decent case ($60+) and voilà- the lovely sound of silence.