Alright, I just had a thought of a new way I'm going to try to position my fans next time I get an old case to mod.
Since hot air rises, I'm thinking of having the only exhaust at the top. Possibly/probably 2x120mm blowholes one in front of the other.
Now all other fans will be intake, yet will be mounted on an angle to help the airflow rise up to the top.
Also there would bea farily large floor fan as well, probably around 92mm.
I would also mod the power supply to how the way I have mine now which is working quite well for cooling it seems. This includes switching the back exhaust PSU fan to an intake and cutting an exhaust hole in the top of the psu and in the top of the case as well. The if you have to, cover the open grill on the front of the PSU. This way the heat PSU has no affect whatsoever on your case or CPU temps.
Below is a simple MS Paint diagram to show what I mean.
It's just something I thought of but had not yet seen done, so it may work, it may not.
Next time I get an old comp to mod, I'm definatly going to try it out.