I wouldn't bother using a filter - unless there is that big of a problem with dust. They just add restriction and noise to the situation, and no one wants that :p
I don't use anything. My PC doesn't collect dust very well as it is in the basement, and I am practically the only one down there - and thats not even that often. I just use some comprassed air and a vacuum once every couple months.
Even then it's not that bad.
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