Look on some site, such as www.newegg.com,
and look at the fans. All the decent ones will list noise level in dBA and air flow in CFM. CFM stands for cubic feet per minute. This is how much air is theoretically moved by the fan. RPM is revolutions per minute, which is nominal and means nothing by itself. For example, given 2 fans, 100 rpm and 10,000 rpm, you have no idea which one is moving more air. What if the 100rpm fan is a jet turbine engine, and the 10,000rpm fan is a 8cm case fan?
Basically, look at *bigger* fans for less noise, because bigger fans (like the jet engine example) can move more air at lower RPM. RPM is usually the main contributor to noise.
If the noise really pisses you off, the only thing you can resort to is water cooling. This can be $200+ and time consuming, as well as dangerous if you don't know what you're doing.