several cases on the market are outright wrongly designed...
what you want is:
1. one large fan (12 cm so) on the bottom front, sucking in cold air, you may add a constant speed controller (zallman) or perhaps a temperature controlled (like those provided by the silentmaxx power supplies)
2. you may consider building a custom bracket directly behind that big fan on the bottom of your case, and mount your harddrives there (it's what i did
)
3. one power supply backside nearby the top, it doesn't matter if it's a two fan, single fan, with the fan on the back or bottom side
(to be honest, it's probably best if you would use a psu with one fan and a CLOSED bottomside of the psu, but only if no. 4 applies)
4. lots of perforation in the case, roughly at psu height, enough to lead all warm air out (not too low, that would not help at all)
5. a 'fanduct' (a little pipe) leading fresh air directly from the side of the case onto the cpufan may help, but it will make the system noisier and doesn't work with all cooler models, it's not essential
6. if necessary, close of all other holes in the case, except the ones near the top
now, cold air is sucked into the bottom, and builds up a 'column' of cold air, en passant cooling the harddrives
the slightly warmer air reaches cpu height, heats up a little more, then falls out of the case thru the perforation / ventilation holes
the psu fan is necessary for cooling the psu, NOT for cooling the cpu, sucking out HOT cpu air to cool your psu is not gonna' help much, ie. the fans of the psu are going to run even faster
if the cpu gets too hot give the big fan in front a little more juice, it's gonna' be more silent than the psu fan anyway, in fact this configuration may even allow you to use one of those 'fanless' or 'semi fanless' psu's
do NOT mount your drives on top of each other, they are going to overheat, leave at least 2 centimetres or so between them, and try to put them somewhere in the airflow...
i'll post some images of my case later, i just tested it with an amd64 3000 1024 gb 3 harddrives (of which one hot wd raptor) and temperature of the cpu stayed around 40..45 degrees C on maximal load, harddisks ranged from 25 to 39 on maximal load, at a room temperature of approx. 20 degrees
last year summer was pretty hot, i wonder what temperatures will show up this year...
