On a serious note, make sure you get a fan that has the same RPM rating or you could damage your machine by under-speccing the fan.
Actually the key number is CFM. There can be two fans running the same RPM but one moving significantly less air because of blade pitch.
Yes, I agree. CFM is key, but you usually see those numbers close to each other as a ratio. Unless you have a large fan, you will likely see the higher CFM fans are Higher RPMs. I upgraded my Avalons with some beefy server fans (4400 RPM) and have seen stable and higher hash-rates.