It depends what you want. If you want a really powerful fan (220cfm), a delta at 4000rpm will be best. On the other hand, that will be louder than a hair dryer. A pair of noctuas will be quiet, but relatively low air flow. I've been using gentle typhoons at 1450rpm, which seem to do the job for radiators. Its all a matter of personal preference, do you need silence or cooling? What are the temperatures like?