my dell at work is almost absolutely silent. it has one large fan with an air duct to the processor, and a fan on the power supply. no fan on the huge processor heatsink. i can really only hear it if i put my head right next to the case.

my PC at home is a different story. two fans mounted to the case walls. fan on the processor. fan on the power supply. it's louder than what would be ideal, but it doesn't really bother me.

however, i just put together a PC for my girlfriend's brother, with an almost identical fan setup, and his is much quieter. i think the reason why is his fan ports are less obstructed than mine.