The more fresh air you have comming into the box the better. Air you bring in must(better) be brought out at the same speed or with the same mm fan remeber hot air rises so the fan taking air out of the box should be at the top of your case. Now back to question are you asking if you should attach your fan to your heatsink?

The answer is yes. As well as here is a site that you can buy some items if you wish. I have done business with them before. Nice Products.
http://www.3dcool.com

Are you talking about a table fan or an actual processor fan?I'm not sure but my casing is sealed up and there's only one cpu fan.I don't even know whether there's enough room for another fan ( as large as the one opposite to my harddisk) can fit in my mobo....like,I want a fan for my graphics card slot but how to attach them?

i made hole on my panel that puts the fan in same area as the cpu and i hooked up a case fan and i had it blow in on the processor and i get cooler temps know then i did before
temps before fan
33c at idle
37c at full load

temps when fan is in place

29c at idle
34c at full load

When you speak of panel I dont know exactly where your talking about?Considering so many types of box and motherboard designs come into mind.Take some pictures of it and post it.

i don't have a camera its the panal that you can open to get intside your computer

Ok so the fan has access to the outside & inside thats good thats what you want.Now how far is the fan away from the heatsink?

side panell case fans are good ,when fan cooling a computer you have to be carfull not to create a small vacum in the center of the case ,most cpu are in the center ,the in and out air flow is inportant ..Big B makes reference to it in post #2

i would say its about 2 to 4 inch's away from the cpu fan i have 2 80mm fans blowing out the back of the case behind the cpu