As Dman stated you want to improve the airflow, intake and exhaust pretty much balanced and if at all possible using a 120 or 140mm fan as this will keep the noise down while providing enough airflow to keep your components cool and prolong their life. You mentioned money but if it is at all possible you may be best replacing the case with a newer type that uses high CFM (larger) fans with lower dBA (noise) emitted.
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