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 heatsink fins, so you get best cooling after your replacement job.
Actual air flow delivered by a fan depends on the resistance to flow due to obstructions in the flow path. This is measured by backpressure - that is, the effective pressure at the fan discharge that results from the air flow resistance downstream. You get lower actual air flow with more resistance. The specs of a fan tell you the max air flow it will deliver at max speed with NO air flow resistance (backpressure), and the max backpresssure the fan can still deliver some (reduced) air flow. That's the Pressure spec - any backpressure more than this spec and the fan delivers virtually no air flow. So you want this spec to be higher, too.
Common CASE vent fans (often labelled as high FLOW fans) will have pressure ratings of 1 to 2 mm of water. But for a CPU heatsink, you want a fan labelled as a PRESSURE fan, with a rating for pressure over 2 mm of water. PLUS, of course, you want a fan with high max air flow. So examine those two specs carefully in choosing your replacement unit. Of secondary importance are the 3-pin versus 4-pin issue (as CountMike says, usually you can set your mobo header configuration for the fan you have) and the max fan noise in dBA.