as your temperatures show, a fan on the side of a PC case is a very bad idea. The exhaust fan should be at the top of the case, and it helps slightly to combine it with an intake fan at the bottom front of the case.
This article is based on actual experiments and is a good read:
You also need dust filters to limit the vacuum cleaner effect - you might be able to hack your existing case by adding a fan at the top and adding dust filters while making sure air enters the case at the bottom front only, lots of duct tape to go with your doorbell power button, making the whole thing very MacGyver indeed :-) If you are in the market for a new case I recommend the antec P182 or similar - excellent airflow and low noise.
edit: after reading mi1ez reply, I must admit I hadn't thought about those tripple GPU setups... I was wondering: might it be a good idea to reverse the side fan to blow air into the case over the GPU's? It's worth a try - just remove the fan and flip it around.
I'm quite certain that a 25 cm exhaust fan will always disrupt tme "more normal" airflow from front to back, as the 70 / 80 degree load temps on his setup clearly show. The large side fan clearly exhausts the cold air before it reaches the CPU / GPU.
Funny, its blowing into the case. It's my main intake. At lowest RPM (400) its already so powerful that it blows against my front intake fan. So now I have air coming from all holes in my computer. As for doorbell button, I've got it connected with speaker wire, which goes through a hole in the back of the computer. I have the actuall button screwed to the wall, which is quite confusing for many people. (has a little "bell" symbol on it)
Antec p180. A case I've always dreamt of. coincidence that you should bring up my dream case.
Unfortunately I live in the middle east, and it is quite hard to get parts here. To get a specific case my best bet would be amazon, and let it be imported. I've brought my computer over as hand luggage from the netherlands. hihi, That was fun. Got a lot of comments. (I was carrying my computer through the airport )
My side fan (which blows in) has been disabled since the day I bought the case.
Loud little thing, completely worthless, pointless, and mostly a gimmick to catch your eye when buying, I think.
If you wanted a side fan, there is plenty, way plenty of room to put a large fan, that would be quiet, and move a lot more air.
Side fan is not a bad idea, it helps to cool GPU, RAM, components on m/b(chipsets, etc). 25CM fan s run at low rpm therefor do not make much noise, at low rpm's they do not give any noticable pressure and cannot possibly blow against smaller higher rpm fans which produce more pressure. Keep one 120mm fan in front for cooling hdd's and put 1 or better 2 fans at the back/top plus PSU fan blowing out. You will need to have negative pressure in the case if You do not count side fan. If You will have positive preesure already before side fan, it will do nothing running @400 rpm as it cannot produce enough pressure to overcome pressure already in case, it will just stir air in itself.
Now for the dust problem. Lift up the case from floor, the higher You get it the less dust will enter the case. Add filters to intake fans. You can make the Yourself from pantyhose, they do filtering quite well but are quite restrictive and clog up quite soon and need to be cleaned regulary. I got fed up with them so I tried something else. I put fine alluminium mesh like this one: http://www.halfords.com/webapp/wcs/stores/servlet/produ... in front of fans and it works quite well. Less restrictive to airflow, easier to clean, probably not filtering as well but case still stays clean. Pet hair and fabric fibre stucks in the mesh and small dust particle, fatty deposits( from sweat ond kitchen if it connected to room) and tar(if You smoke in the room) sticks to that hair and fibers on the mesh. Part of the small dust particles enter the case but are blown out as well because since there are no fibre, fat,tar in the case to stick to.