With a blower type graphics card, I don't think it matters very much whether you arrange the fans in your case to provide positive, neutral or negative pressure inside your case, since the blower should be designed to move enough air through the heatsink and out of the case to adequately cool the graphics card.
It is possible that a positive pressure cooling arrangement might provide a slight advantage, since your graphics card fan is working with rather than against the pressure differential, but I that imagine the benefit is minimal.
With a graphics card which uses an open cooling system, where some of the air may be exhausted from the case, but where some (or all) air is circulated back into the case I wonder which cooling arrangement will give the most efficient cooling (to minimise power, noise and component temperatures).
From the research I have done so far, it seems that blower type graphics cards are recommended for a positive pressure cooling situation, while negative pressure cooling is recommended for open cooling graphics cards, but I'm not so sure.
For example, lets consider a case with inlet fans at the front and outlet fans at the back (ignoring the added complication of side or top fan mounts):
------------ ------------ < < < | < < < | Out < <CPU< | Out < <CPU< | < < < | < < < | | < | < <<<GFX<<< < In <<<GFX>>> < In | ^^^ < | ^^^ < | | | | ------------ ------------ Blower design GFX Open design GFX
With an open design graphics card and positive pressure cooling, the <<< flow may be increased due to the advantageous pressure differential, but the >>> flow may be decreased due to the incoming air from the In fan.
With negative pressure cooling, the <<< flow may be decreased due to the disadvantageous pressure differential, but the >>> flow may still be decreased due to the incoming air from the In fan.
From an intuitive perspective, I wonder if a neutral pressure arrangement (where intake and outflow fans are balanced) might not be a better option with an open design graphics card.
I also wonder if there would be a significant advantage from adding internal baffles, such that airflow from the front fan is directed up to the CPU and down to the graphics card intake, but away from the recirculating air of the graphics card:
------------ < < < | Out < <CPU< | < < <_ | | \ < <<<GFX>>> >< In | ^^^ _/ < | | ------------
I know that a few years ago, case manufacturers went through a phase of promoting cases with airflow zones, but such cases never seemed to have a significant advantage (in noise/thermals) over un-zoned cases, so I suspect the effect might not be significant.
It is also possible that a positive positive pressure cooling, with vented blanks close to the graphics card could encourage the recirculated air to leave the case sooner rather than later.
Unfortunately, I don't know enough about the physics of cooling or fluid dynamics (laminar vs. turbulent flow etc.) to be able to assess the likely effect of these options. In my research, the closest I could find was a brief mention of the effect of open graphics cards on cooling in a Toms Hardware Guide article and that doesn't offer any concrete advice on this configuration.
So, overall, which airflow design option, if any, is likely to provide the best balance of thermals, noise and power when using a single open cooled graphics card?
- How To: Properly Plan And Pick Parts For An Air-Cooled PC, Part 1 and Part 2 (Toms hardware)
- The Big Air Cooling Investigation (bit-tech)