hey i hae that same case....aspire x-dreamer? anyways, i put two 80mm intake fans up front and a 120mm exhaust in the back. get some really quiet 80mm fans though because those spots up front are rather blocked off and will make alot of noise due to restricted airflow. but you definitely need two, or hardly any air will get sucked in. do you have the intake fan on the side panel? if so, you might want to replace that with a quieter one too, it's very loud. but it is very important and gets ALOT of air to the components.
in my experience, usually, whatever side of the fan the wires are coming out from, is the direction of the air flow. you can aslo look at the shape of the fan blades and see the most efficient way for them to turn and use that to determine air flow direction.
Intel E8400 @ 3.78ghz
8gb (4x2gb) G.SKILL DDR2 1600 @ 420mhz, 5-5-5-12
ASrock P43Twins1600 (hates RAM dividers)
WD Caviar 500gb + 250gb hdd, 32mb cache
eVGA 7900gs @ 585/1700mhz
OCZ GameXStream 600w PSU
Sound Blaster X-Fi Platinum
Cooler Master Centurion 5 case