If using a cool tube theres no reason to exit out the back, and it might be too wide to get a couple of 90's in there - Ideally you would want the intake to come in and exhaust out the sides of the cabinet - a straight shot through the tube with a fan on either end - then a couple of PC fans might work, but not if there are any turns..

I don't think the PC fans are gonna cut it for you - plus they will be loud. If you want a powerful, virtually silent fan, check out the Panasonic Whisperline models. The only thing I hear is the air moving in the duct, the fan makes hardly any noise. A couple of 120mm fans will be much louder.