the ambient air temp does play a role in the temps of the comps components. Best thing you can do, is find a fan with the highest CFM, at the lowest DBA level for the amount you want to pay for the fan(s).
Basically it depends on what you are willing to do. If your computer is running just a little hot but is still stable then the choice becomes one of preference. If it's so hot that your CPU is becoming unstable then you really don't have much choice and probably have to upgrade your HSF.
It was almost 90 where I live today but that was outside... my room was probably only 80 or so... I had no issues with temperature but then again I'm going to be switching to water cooling this month as a matter of both being able to OC and lower the sound.