A few years ago my very expensive 8800 Ultra graphics card bit the dust. And I mean literally, it was totally clogged. When I rebuilt my machine over the summer I got a new case, a Silverstone Raven III. The airflow is vertical with two massive 18cm fans on the bottom covered by washable magnetic dust filters which are removed laterally, not from the inaccessible rear like most cases. The drive bay slots all have washable dust filters too. For the remaining 12cm intakes on the side and rear I picked up some of these Silverstone magnetic filters to stick over the intakes. I live in a clean house with no pets, but find the filters are completely clogged after only three months.
Anyway, the interior of the machine is still pristine 9 months on. Just wanted to make my pitch for proper filtration.