Positive pressure is useful for keeping dust out if you have filters, but I've found that in cases that have just one fan, blowing out results in significantly better temperatures than one blowing in, and in a case with two, one in and one out is best.
If your case is on the floor, you don't want fans on the bottom blowing in (because of dust/hair).
Regardless, I'd have two on top blowing in, and two on the bottom blowing out. You definitely want a fan near the video card doing something, as it's doubtless the largest source of heat in your case, and having that air immediately removed (and in a direction that discourages dust from entering your case) seems ideal.