put two 8 cm fan.. 1 near ot back end corner to the HSF and one near to the cd drive..because the heat may circulated around the cd drive area if the 12cm fan blow through the gpu and psu...
But one of the problem is ur psu is beside the gpu and hdd....all of the unit generate heat...maybe u if u can move the psu to front and the 12cm fan to back..this will maintain the flow of air direct to the gpu and hdd...

well.. that a problem but try to get a longer power cord...or putting a another fan at the middle facing toward the gpu and hdd. i think u can build it by attaching the fan 1 side to the cd drive support column....

by the way..is there are holes around the back casing..?
if there are holes then maybe u can build a external fan screwed to the holes at back there and put one 9cm fan at other side...


below are the pic i edit...

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