dude....u wan dust free... ur gonna have to go with positive pressure...

put lots of FILTERED intake fans and mebbe only 1 exhaust fan (or 0..let ur PSU be the only exhaust)

at least this way ur dust intake will be concentrated on the filters.......which u can clean. although take note that filters dont stop dust from going in...it only minimizes the amount of dust

if u put negative pressure, u have no way of controlling ur dust intake. the dust will be sucked in from all nooks and crannies of the case... even ur DVDRom slot.