I can't recommend replacing that fan when a good after-market cooler is so cheap. 24.9939.99
Replacing the stock fan only solves half of the issues I have with stock HSFs.
While you're at it, you might as well replace the HSF too. Invest a little more $ and an hour of your time for a better pay off.
Or let me put it this way, if you're replacing the fan because of its loudness, you'll have to go slower. There's a lot improving build quality/design can do, but you're still up against raw physics.
Now if that new fan happens to move less air than the old one, and you stick it in a heat sink that's designed to be cooled by a 6000rpm fan, what's going to happen to the temperatures?
I'm almost willing to guarantee you that you can replace the stock fan with something that's a. slower, b. quieter, and c. better.... only to find your cooling performance is held back by the heat sink.