Additionally, positive pressure helps keep dust out of the case as long as the intake fans have filters.
Not sure I understand your logic here Spartacus...It shouldn't make a big difference if you have a positive or negative pressure. Most PC cases don't have too many open spaces that lots of dust will come in. Not to mention that it would take a very negative pressure to create enough air flow through a crack, hole etc to suck much dust in.
IMO, lots of cool air flowing through the case and around your hot components is the key. You want your in and out to be balanced so that you have a neutral pressure in your case. Ideally, I think you want a neutral pressure so that your fans aren't fighting each other, thus giving optimal airflow.
Also, you should try to set up your fans so that all the air flows one direction. You don't want two front fans pulling air in, just to be pushed out by the third front fan.
Also, keep in mind that hot air rises, so pull air in at the bottom, exhaust out the top. Generally, you also should pull in cool air from the front to blow over cooler components like the HDDs etc and then get exhausted out the back as it warms up blowing over the CPU etc.
Depending on your GPU heatsink (does it exhaust out the back of the case or just back into the case?) you'll want a side fan blowing cool air on it, or pulling hot air away from it (also depends on how many other fans you have pulling air in).
In my rig, I have a single 120mm front fan and a 250mm side fan blowing air in. The 120mm front fan blows cool air on the HDD. The side fan blows cool air all over the mobo, CPU, GPU. Pulling air out is my 120mm exhaust fan, a 140mm fan on my PSU, and my GPU HSF. Pretty sure I have a negative pressure in my case and I don't have problems with dust at all (mostly cat hair sticking to my fan grills of my intake fans
Compare that to an old dell that has a single fan exhausting (and PSU fan exhaust) and no grill or filter over the intake. The damn thing collects tons of dust. Moral of the story-use dust filters, fan grilles etc.
Also, they sell PCI fans that fit in a PCI or PCI-E slot in your mobo and can blow cool air or suck out warm air through an addon card slot.