All above is good, but there's more to consider.
Two things are important in the specs for a fan used on a CPU heatsink: Air Flow and Pressure. Both of these are usually shown as max values in fan specs.
The max AIR FLOW you want to be higher -that's the maximum flow (and hence best heat removal) when the fan is running full speed, and it really is only achieved when the fan has zero backpressure (resistance to air flow). In reality the max air flow you do get is less, because the narrow passages in the heatsink do offer flow resistance. FYI, if you are replacing the fan on the heatsink, but not replacing the heatsink itself (that is the best path), while you have the old fan off be sure to clean out any dust accumulated between the...