Corsair Carbide 400r positive air flow

I am getting a new Gigabyte gtx 670 Windforce OC and wanted to add more fans to my system. I bought 2 of these NZXT Fans and was wondering how I should set them up. My main concern is keeping dust out of my system (ie. setting up positive airflow) and overclocking the GPU. I have 2 mounts on top and 2 on the side and a bottom one, and I left the stock fans alone. I also have a Hyper 212 cpu cooler pushing the air toward the exhaust fan in the back.

1. I was thinking of putting one on top as intake with a fan filter and one on the side by the GPU as exhaust, since the Windforce pushes all the air downward. I heard that side fans are usually intake so is this a bad idea? How about putting both fans on the side, one intake on the top side, outtake on the bottom side.

2. I was also considering blocking my unused fan mounts so dust does not get in. Is this a good idea? I don't think the ones on top matter so much but will blocking the side hurt exhaust from coming out? What should I use to block them, plastic wrap, thin foam, something else?

I don't have the gpu yet so I can't really test anything yet.