ahhh, the bottom mounted I have means i can direct the hot air from inside the psu straight out the base of the case whereas the top mounted ones blast it into the case. good thing is the current fan in it is mounted to the 5v rail =] so you can just switch it out
i havent opened myne up yet, i shall have a look when i have enough dough to get a new fan, i just reckon ill end up hammering in a delta and ussing it as a exhaust