I have another thread below. I do not mean to spam here.
Just a quick question. If the fan is a big 120mm fan on the top of the PSU, does that mean I have to have a bottom mounted PSU spot. There are both ones that have 80mm rear fans and ones that have the fan on top. What is the difference?
Just mount the PSU upside down if you have to. Mine is upside down in a bottom mounted case, because the grill was hitting the supports. Should be fine even up the right way, as there should be clearance between the top of the case and the top of the PSU.
Bigger fans usually provide more flow for speed. So a PSU with a 120 or 140mm fan will normally make less noise and cool itself better if it needs to. Often 120mm or larger fans don't fit on the back, hence the top/bottom placement. My 650W unit uses a 140mm top mount, its completely silent, although the silence is ruined by my graphics and CPU coolers
the 120 mm fan will work fine in both top and bottom PSU mount cases, in top mounted cases the fan points downward, in bottom mounted cases it points upward, in both cases it draws in air from the case and serves as another exhaust fan. A 120mm fan will move more air with less noise than an 80mm fan in the back, which is why more and more PSUs are using them.