Help adding fans please?

[IluvWin7]

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(3-4 mm gap). 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.

 

[vigilante212]

Most people run fans so it blows air in from the front and sides and exhausts through the top and rear.