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?product_id=0341409
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.