if you get one that reaches back far enough to align with the gpu then it aids in getting the air coming off the gpu out of the case, and can help add rpm to the gpu fan.
that is if the gpu fan is pulling air through the hs, then both fans are working in the same direction.
a friend of mine told me that is what he was doing. i was suprised to find out that his gpu fan was not blowing air onto the hs, but the other way around. didn't really believe him at first. come to think of it, i never even checked my own to see what it's orientation is.
hold on a second...............