Hey H,
I do have an extra 120cm fan that is mounted in the upper chamber. I ALSO don't have any hdd's there. I modded my wires so when I open the side case, there is NOTHING in the path of the vid cards. I have the hdds in the bottom cage for heat dispersion. AT first I put them there because I wanted to make my case more quiet, but when my rig is purring, I put my ear to the drives and I can't hear anything. I DO KNOW that the maxtors get hot when there working so I would rather the bottom air get flushed out the back through the PSU fan.
I have a 120cm fan on the roof, one at the the back, one in front of the empty hdd cage, and one in the bottom chamber halfway between the hdd's and PSU. This way it sucks the heat away from the drives, and the PSU fan helps push it out the back.
I use the optional mobo fan near the CPU cuz it helps cool the rad that dissipates the heat from the north and south bridge. Because the heat rises, it travels through the heat pipes, and up to that rad. DAMN WHAT A NICE SET OF COPPER PIPES THEY ARE 8) (chuckles maniacally). When I brought my rig in for a check up on other issues, the tech suggested I use it given the vga and mobo temps. Believe me I don't even hear it.
I think if you want to add a fan in front of the lower cage compartment, you will have to mod the shroud due to lack of space. However, I really don't think you would need it because the mid compartment fan takes care o that.
When you mention the PSU fan direction, you mentioned yours is facing the side. How can that be ? If you have the S12 600watt, then the fan is either at the top or the bottom depending how you install it. I put mine in facing the up because there is more breathing room there. take a look at your case and you'll see what I mean. And the fan acts as an EXHAUST. If you think about it, the PSU gets hot like a car's motor so it needs an exhaust to get rid of the heat right.
If ya got more questions fire away and I'll get back to ya.
cya
MIke