My G5 build.

[minihack]

Understood. I was referring to the hot air that actually makes it out of the case. Convection would cause it to rise into the path of air entering the machine.

Ah, right. Of course, you're right about that... Unfortunately, I want to use the original rear fan assembly, which forces me to move the board further up; this in turn forces me to place the PSU someplace else.

You could always have ALL the fans sending air from the back to the front. It's not typical, but is sometimes configured that way. Lian Li had a case that was thought to be better by having a front mounted PSU and with air flowing back to front:
http://www.tomshardware.co.uk/lian-li-pc-a05n-aluminum-case-chassis,news-30128.html
...just a thought.

 

[flatfoot]

Hm, I think I'll rather put the PSU below the CPU cooler then and make a shroud/plate to separate the PSU's intake from the CPU cooler.
When I replace the Thermalright Silver Arrow with a Corsair H100 I'll be able to leave it there anyway, so there's no point rethinking the airflow in the case. I also already put new fans into the rear fan assembly, which would be a PITA to take out again and turn them around.

Thanks a lot for your thoughts anyway!