Well there are a few Fans in a PC so It depends on what fan you want to replace....
The 2 main Fans are in your Power supply and one on your CPU ,In my experience they are both equally as Loud...You can"t just replace the one in your Power supply without also replaceing the power supply so for that it would be best to get a Quiet power supply or a Nice passive one that has no fan like maybe a Zalman.....
As for your CPU Fan you might also consider getting a Passive Heatsink with no fan ,That way your PC is totally silent as there are no fans to cause noise as all fans will make some noise so elimateing the Fans them selves is the best way to eliminate the noise.....
Take a Look at the "Zalman" web site as they sell some of the Best Passive CPU Coolers and Power supplies which will eliminate most if not all PC noise (Minus Drive noise ect)....
Cheers