PC fan powering question

Im wondering if anyone could answer 2 questions for me im building a box and just went out and got an enercell AC adapter from radio shack that runs either 9v/10.2v/12v/13v along with an 80mm PC fan to cool a 5 cubic foot box first question is, the fan puts out 28cfm is that enough to cool the box? second question is i see people talking about DC adapters, now switching between all the volts it doesn't really seem to change the speed of the fan is that because i need a dc adapter rather than ac adapter is there a difference or should i just simply get a bigger fan like 140mm fan if anyone could give me some insight i would be very gratefull thanks

Missing information, whats the size of your box or cabinet? You want to move the amount of air within your cab at minimum once every 3 minutes and of course CFM's are cubic feet per minute so easy to calculate there, but the smaller the area the more cfm's you want because of heat buildup. And finally you do realize that for healthy stems it is best to also have a fan blowing on your plant(s) directly and another to exhaust?