Side case fan blowing In or Out???

Side fans are alway intake for all the systems I build. Hot air will rise so the best place for exhaust fans are near the top of most cases. Ideally a top fan is the best exhaust location along with the back location esp if you have front intakes. This will create a cross flow thru the case. Air movement is very important in air cooled systems. Even if there is not a top fan exhaust, if you select the right PSU, it will draw the air out thru its exhaust. Just a note, the cases are not air tight so hot air will always find a way out.

But it is always necessary to create negative pressure in a case -i.e. more air being taken out, then being taken in. Depending on where the vents on your case are located you should make your fan an exhaust -as long as their is cool air being taken in from somewhere that is pulled over the hot component.

Only if you can create an air tight environment. But all cases leak so hot air will always find a way out. Its much better to bring in more cold air since its harder to do then trying to get out hot. Take the example with CPU HS's. They work better when the air is blow down/across the chip then when they are sucked up or thru. Also if you bring enuf cold air in it will force the hot air to the top where either a top fan or a top mounted PSU will pull the hot air out. Also concerning side fans. I tried both in/out to cool a SB chip or esp a vid card, the air coming in works a lot better then trying to draw the air out thru the side. Rem that in the case of a 8800GTX/GTS it exhaust air out thru the back slots also. The chance of the side fan being strong enuf or large enuf to actually draw enuf air side ways (short of using a very powerfull or large fan) is not as easy as just drawing cold air in. But thats just IMO...

considering their is an exhaust fan at the back of the case at a similar height, I would direct the side fan blowing IN.

If your CPU fan is close to where that side case fan is, then you should take into account which way your CPU fan is blowing. You don't want them working against each other...