Case fans ? Air in or out (Case suggestions ?)

Question

Maybe this is a stupid question.
But because of my high CPU temps i was beginning to think that maybe i had installed all my fans the wrong way 
About case fans. Should they be installed so they draw cold air into the case or push hot air out ?
I ask because i was in another Dansih forum where thay was talking about a new case with a 125cm (Big) slow fan in the side,
and the guys said it should draw cold air into the case instead of pushing hot air out...  mmm
Normally hot air should be pushed out, right ???  or
Second. How can you if the fan blow or sucks air ?

Answer 1

I have a windowed case, so when I see some dust in the window, I do open the case and clean everything that have dust, and this is every 2 weeks, or less... :D
I understand that if you have overpressure inside the case and use filters with every fan, in the same period of time you will end with less dust inside than with underpressure, because the air will not enter through every hole of the case, but if you do not use filters (as I don't) the resulting dust in the case will be more or less the same.
I don't like filters because they greatly reduce the airflow produced by the fans, so I prefer to clean the case more often.
And about "getting as much hot air out of the box as possible", I have to say that I've tried with a big 38 x 120 mm rear exhaust fan of 103 CFM (the noise was unbereable !!) and the case temperature was the same as with the stock 25 x 120 mm fan that has much lower CFM, so every solution has its limits. I think that is very important to extract the hot air from the case, but nothing is more efficient than a direct and fresh airflow over the device to cool. I have 2 x 80 mm front fans that flows directly over my two hard drives, and they are currently at 29 and 24 ºC (the Samsung is amazingly cool), and even in summer the temperatures are barely higher.
Anyway is very difficult to find the best setup with a given case/fans configuration. I have another 80m fan in the side window, and when I've inverted the airflow (stock was in exhaust mode) the processor temperature dropped 2 ºC, but in the same time the motherboard temperature unexpectedly raised 2 ºC. I leave the side fan flowing air inside the case, but I can't understand why the motherboard temperature is higher.