One note on using the adapter approach - this is a great idea as you do not need the multiple voltages of a PC
supply nor the power. However, when picking a powerpack be sure that it can provide enough power (it's rating in amps or watts should be at least 20% higher than what the fan is rated at. To convert amps to watts use this formula: watts = amps * volts, to go the other way use amps = watts/volts. Becareful with units as a mA is 1/1000th of an amp.\
Good site to get Power packs and powerful fans:
Also, as for fan's ability to move air - compare the fan's CFM (Cubic Feet per Minute), for example a 20CFM fan moves alot less air then a 40CFM fan.
Recall that if you are using two fans and they are on opposite sides of the hood they should function in reverse (assuming you are not using anyother venting). For example, one on the right of the hood blows in the one on the left blows out. This will create the greatest cross ventalation.