all fans have one very important rating called "cfm".it tells about the amount of air moved by fan & in real life this value is usually in +/-10% range.a 90cfm 120mm fan will much more air than 45cfm 120mm fan but it will also be costlier.if you have a lot of hard disks then installing a higher cfm fan in front is better but if you have only 1-2 hard disks then installing any average cfm 120mm fan in rear should be fine.for cpu cooling unless you are always using 80-90% of processor even the stock heat sink(fan which comes with cpu)+a good cfm rear fan is sufficient for 1-2 hard disk & no graphics card setup.for a mid range graphics card add another 120mm fan.considering all this if you have the budget get this 90cfm 120mm fan:
Cooler Master SickleFlow Blue 120 mm Cooler - Cooler Master: Flipkart.com
also no fan will help if you have the usual intex/zebronics/iball cabinet.at least get a cheap cabinet from coolermaster to effectively use such fans.80mm fan is really not worth it in long run.
Thnx a ton bro... this is the kind of info i was expecting.
Right now i have Intex cabinet, 1 80gb HDD, no GPU. So for this i m considering 80mm fan (as i checked screw distance, bigger one is not more than 90 mm).
Now after upgrade... NZXT gamma, 1 1TB HDD, Sapphire HD7770.
1)So for it a 120mm NZXT fan already comes with cabby, this new brought 80mm & another 120mm should be enough??
2)Or 120 (NZXT) + 120 extra will do the work(hence i can ditch this 80mm)????
3)Which brand should i go with, Cooler master, Bitfenix and Deepcool??
4)Diff b/w these two 120mm CM fans Cooler Master 90 CFM LED Cooler - Cooler Master: Flipkart.com
& Cooler Master SickleFlow Blue 120 mm Cooler - Cooler Master: Flipkart.com
(earlier one is cheap)