A well-made fan @ 2500 RPMs can have more CFMs (airflow) than a poorly design fan @ 3500 RPMs. And no one can tell you what you need because the ambient temperature of your case, home and geographical region will vary. If you want to use a slower fan, get a better, bigger heatsink (Stock Intel heatsink isn't that great) and get some good thermal compound.
Your stock fan should be 'good enough' and if it isn't, clean the dust from your heatsink/fan and reseat it with some fresh thermal paste.
Hope this answers your query!
Happy weekend! 