Amdmb.com has an excellent technical guide to fans used in PCs. Note that this article is technical in nature - it won't tell you 'buy xxx fan', but it will give you a great understanding of how fans operate, what their ratings really mean in real life, and where the noise comes from...
http://www.amdmb.com/article-display.php?ArticleID=132
http://www.amdmb.com/article-display.php?ArticleID=132
