The CFM isnt exactly the best way to measure it, as 90% of companies over exaggerate by about 2:1 on the fans. Also, what size fan was it before, what PSU do you have, and what fan did you have before? You might want to buy a higher pressure fan, like made for dense heatsinks. This:http://www.performance-pcs.com/catalog/index.php?main_page=product_info&products_id=28136
might make for a good PSU fan, although i wouldnt know.