I have my PC facing an open window, it gets cooler air than I do... mainly as it faces away from me, and is between me and the window (for a long time, every unknown noise was thought to possibly be the PC and I would have to get up and walk to window to make sure noise was outside and not PC. I am used to it now). The distance between an open window and front of the PC is only a few feet. Nothing blocking the air besides my curtains.
I know that really has nothing to do with dust but it means its getting the freshest air in my room so it helps.
I have my PC on a box to keep it off floor, another way to stop dust getting sucked in. Also helps with air flow.
Its 2 years old and I have only had the front off it once to clean dust from that filter. I have more exhaust fans than intakes so really, dust builds up on the exhaust fans and the filter on intake fans has an easy life. Probably need to blow some air through my radiator. Pretty easy to clean the exhausts too, just take top of case off.