I have some crazy designs like 5 120mm Delta Electronics fans and two 450W PSUs that have their own 92mm fans all the way to a system with the side off and a 200mm two speed house fan sitting next to the components.
Basic designs are the fan intake in the bottom front and exhaust fans in the top rear. Other use blowholes out the top and intakes all over. Most designs are fine. You will not get huge system performance improvements because of fan placement. Unless you are talking about no cooling verses massive cooling. The cooler, but not below the room temp, the better. (Condensation reasons)
see too many cases with bottom front fans, but always have to see where does the air get in there. way too many have almost no way to get decent flow to the fan and you end up circulating the air in the case
like mine, there is space to add 2 80mm front fans, but when you look at it there is barely any space for the air to enter from the outside to the fans (there is a tiny slot on the bottom to let in outside air, imagine the hoover sound the two fans would make trying to pull air in through that).
I cut holes for two 120mm fans on the side cover and it was all good (2 80mm on the back factory), since I replaced my setup (XP out P4 in), now the holes are there to make it look interesting (only running 1 80mm on the back).
well designed cases with decent space for air going to the front fan(s) are cool
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For the best setup you want a good case first.
I got the superflower sf201 and this case is an incredible airflow machine. It has two fans in the front with a huge grille and a coarse sponge-like plastic insert to trap dust (it can be washed). these fans blow a nice strong draft over my HDD, there is a fan directly over the cpu that sucks in air, there are two fans in the back that push air out and there is one on top to push out more heat. then my vantec stealth black aluminum psu (really loud) has 3 fans, back, bottom and front. Aside from the unacceptable volume of the vantec power supply, the rest of the fans barely make a whisper.
It is highly recommended that you have fans in the front of your case, preferably infront of your hdd. If you have insufficient vents, make some. Another great spot is directly over the cpu. The rest of the fans should be in your power supply. you dont really need fans in the back or the top because the power supply should take care of that.
Asus A7N8X ND + Athlong XP 2700(2.17ghz)
Mine is 192fsb x 12 = 2304mhz @ 1.775v
Corsair PC 3500 512 @ 5-3-3-2T
Audigy 2, Western Digital 80GIG SE 8meg cache
and Some CocknBalls.