trying to upgrade my psu, i have a dell 4600 250wpsu. i would like to upgrade my psu to a 450w, but i dont really know how to buy one. my current psu vents out the rear of the pc and the plug is obviously in the rear as well. i dont know where the air is being vented, i suppose its inside the pc, then vented out right below from the pc's fan. so in buying a new psu, do i just look for one that only has one intake fan from the rear???? should i buy one that has multiple fans even though i dont know how its being vented (still assuming its from the pc fan below the psu) and do i absouletly have to buy one with a fan from the rear???? thank you