lol
I don't have an antec900, but ive got a thermaltake kandalf.
it has similar fan configs, with the exception of the 20cm fan at the top being a 25cm fan on the side panel. within 2 days of lugging that case into my house, the front filkters were clogged.
any case that basically has opened mesh panels/heaps of fans is likely to be a dust gatherer. it does depend too on your home environment, the effectiveness of the filters and the cfm of the fanms