slightly less airflow, but still pretty good. On mine I put the stock fan in the case, and then put 2 silent fans on the EVO to get the flow back with a little less noise than the stock fan.
Bench temps for my i7 2600 are 58-61*c on all 4 cores when clocked at 4.2GHz and the fans all at their lowest settings.
Hook it all up first, you likely don't need another fan. The fans I replaced my system with are some low RPM 120mm Enermax fans found at my local microcenter: http://www.microcenter.com/single_product_results.phtml...
I got them on sale for $10ea at the time. They are extremely quiet, but because of this there very low airflow to them, so you typically need to buy 2 for every 120mm fan you replace. For a normal system there is no need to get fans like these, I got them because I do audio recording from time to time, and these are quiet to begin with, and easy to filter out what little noise they do make.