Windows 7 - Just how many fans do you need for a system?

well i am going to add fans to my computer which is now only running with the stock CPU fan.
I have decided to get a new fan under my hard drive bay and make it blow air directly under the hard drive. Is it right?
next i will like to get a fan on the back. should it be exhausting or in-taking?
Please give detailed answers and reasons.

Back < exhaust, Front < intake
Bottom ^ intake, Top ^ exhaust
Try to balance it out so you have the same amount of fans drawing air in as you do blowing air out.

You just need good case to get excellent air flow. Also 2 fans are enought,but good fans,not anyone...

Personally for me, I will use whatever the case came with and adjust AS NECESSARY.

At present, my box at home runs in an Antec P182 case. This has 3 x 120mm Antec Tri-Cool fans. 1 at the top, 1 at the upper rear, and 1 at the bottom between my hard drive bays and my power supply. On this box, I run an Intel Q9550 with a stock Intel cooler, which is overclocked slightly from 2.83Ghz to 3.40Ghz. My temps are low 30's at idle and around 58C at full load. No need for any additional fans.

My work computer desktop runs in an Antec 300 case. It has 2 Antec Tri-Cool fans. 1 upper 140mm fan and 1 upper rear 120mm fan. This computer is running an Intel Core 2 Duo E8400 with a stock Intel cooler, and is overclocked slightly from 3.0Ghz to 3.6Ghz. My temps are mid to upper 30's and my max is around 55-60C. No need for additional fans.

So, I'm living proof that with decent wire management, stock coolers, minor overclocks and daily used machines...having tons of case fans is not always necessary.

adding fans can add noise so take that into consideration.
If your case permits, get large low RPM fans to limit fan noise.
There are tons of options and price points for the fans.

My current case has 3 230mm fans (top, side, and front) and 2 120mm fans (these 2 are on my Corsair H60 Hydro Cooler)
http://transamws6.com/pics/pc/2010/haf-inside-1.jpg
Above pic was taken right before I installed the second fan on the H60.

to everyone who answered.. thanks!!!!!!!!!
But You didn't see my key point?
What about the hard drive bay fan????????? it is just 3-4 mm away from my harddrive at the bottom and i have to be sure that it won't affect my hard drive...........

PLease answer..

I don't generally find that you need a fan blowing on or across your hard drives. Generally speaking, the front lower part of your case intakes air which is usually naturally drawn across your hard drives. Your exhaust fans in the back pull that air out of the case...and thus pull it in from the front.

For your kind information I don't have any front air intakes

neither do I. But I have ventilation holes in the front of my case. And I have exhaust fans in the back of my case. So, the fans in the back pull the air from somewhere that they push out the back. This air is drawn in naturally through the front ventilation holes and is blown out the back.

If I put my hand in the front lower portion of my Antec 300 case, I can clearly feel the cool air flowing in. In fact, I can put an 8.5x11 sheet of paper on the front of my case and it sticks right to it from the incoming air flow.