By putting a positive pressure in your case and having filters over the inlet fans your case will stay much cleaner. With a negative pressure dust gets drawn in through the cracks. There are all kinds of filter elements you can buy such as that for household heaters, vacuum cleaner bags to car air cleaners to most any sort of air filter element you can find.
Since you only have a 420 watt PSU, which is not much waste heat wise, I would consider two inlet fans with filters and one outlet fan. To test the pressure put a piece of Saran Wrap over a crack and see if it gets drawn in or gets blown away.
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Last edited by David M; 07-10-2011 at 05:42 PM.