Here's a tricky topic.
I've got a Silver Demon case, and it comes with 2 80mm fans.
One is on the side with the glass display; the other is on the back of the case (below the PS).
By default, the fans are set to suck heated air out of the case. However, there's no air flowing in that way.
I turned the side fan around so it will blow air into the case, while the rear fan is still sucking hot air out (although it does feel cool, because the warm air is coming from my dual-fan 500 watt PS, conveniently located over the CPU).
My heating problem is not with my CPU, a 2.4 GHz P4 (which cannot be over clocked because of my MSI SIS MB) that stays cool with the stock Intel jet engine (going to have to get that replaced sometime). My PNY 5900 has a bit of a heating problem. I'm using the stock fan and I'm slightly OC'ing it. It idles at 50C right now. Yeah, PNY's jet engines really suck at cooling, but I'm not sure if I want to get an aftermarket cooler now or wait until I upgrade my video card.
Anyway (ahem), I thought that by turning the side fan around, the cool air would help cool the card off. Before I did this though, just for fun, I took both fans out and pulled the side panels off to see how the Temp changed.
My 5900 idled at 47C when I did this, but you could hear my rig roaring 20 feet away (good thing I use headphones. Did you know those can completely remove the problem of having a noisy computer? Try using stock fans with Sony sport 2 headphones & BF2, all you can hear is stuff blowing up in BF2 because the earphones prevent sound leakage
After I put the panels back on with the side fan backwards, the card's temp went back UP to 50C, instead of staying at 47C. I think this is because the PNY fan is approximately directly underneath the side fan, so it doesn't get hit by the airflow, and the PNY fan is in the under-side of the card.
Would it make a difference if I made both of the fans intake or outtake? Or should I have one intake and the other outtake (that means should I have one blowing air in and one out, or have both do one thing)?