Sorry but I have to disagree with this. There is HUGE variation between different fans, not to go in to specifics of the technical differences but take this as an example:
http://www.scan.co.uk/products/140mm...500rpm-33db(a)
http://www.scan.co.uk/products/140mm...lade-quiet-fan
These are 2 140mm fans, both roughly the same speed (1500/1600RPM). But look at the difference in sound level: 33dB vs 26dB (that's about double the volume), and the CFM 70 vs 110 (again nearly double).
So this example shows that a good fan can give (nearly) double the airflow with half the noise.
The main things to look at when buying a fan is NOT the RPM speed, but look at the sound level and CFM (CFM is cubic feet per minute - i.e. how much air it moves per minute). Then its just down to what you want, silence or high air flow (or a balance).
But remember that the decibel scale is not a straight line (its a log scale), i.e. the difference between 10dB and 20dB is more than double (its about 10 times), same with 20dB to 30dB.
My rule to thumb is:
Noise: 0-30dB is quite/silent, 30-40dB is moderate, 40dB+ is loud (xbox 360 runs at 55dB).
Air Flow: 0-50CFM is low, 50-70CFM is moderate, 70-100CFM is Good, 100CFM+ is great.
p.s. the akasa Viper is one of the few fans that can give high CFM and low noise (I own 2 of the & they're *fan*-tastic).