>>4
The best opinion I can give you is to visit newegg.com and find the best rated for the size/noise/power.
>>3
I'm assuming these fans are external (on the case)? If so, i'll make a safe guess...
Back-Top: Most likely your power-supply fan or motherboard's.
Side: Airflow/Cooling
Back-Bottom (Middle on my computer): Airflow Main/Cooling
If that bottom one never turns on, and is indeed your main fan (should be the largest), then that's a problem.