Well, I suppose you can check out Zalman's line of CPU fans/heatsinks (you're most likely going to have to buy them together). I have this one and can't hear it. Many of them have a speed adjuster so when you're just doing basic desktop stuff you can have it near silent, if audible at all.

What do you mean by motherboard?

And it's not quite recommended that you go replacing your power supply's fan. We want to see you on the board again Zalman sells "silent" power supplies (so they say), like this one.

I know I seem Zalman biased, but the products I've bought from them have been just great. They've served me quite well.