So I did some research and it sounds like my case needs to have positive pressure when it's on to push dust out, rather than creating a vacuum which makes it easy for dust to settle in.
I have a HAF 932, but I took the sidefan off.
Right now it has one 230mm top fan as exhaust, one 140mm rear fan as exhaust, and one 230mm front fan as intake.
I understand that in order to create positive pressure, you would need to have greater combined CFM from the intake fans than from exhaust.
I have a radeon 5970 and an aftermarket Noctua NH-D14 cpu cooler, and I'm not sure exactly how these fans affect airflow. Since the 5970 pushes air out of the case, I assume it functions as an exhaust fan and needs to be countered with more intake. The Noctua pulls in air from the case, but it doesn't directly push it out, either.
Keeping the room free of dust isn't an option. What can I do? How will I know for sure that it has postive airflow?