Expansion slot fans aren't all that good. Putting an exhaust fan beneath a video card would probably starve the GPU's fan and would increase temps (maybe to overheating). Putting one on top of a video card shouldn't really help. Heat may rise but not very much and the GPU's heatsink still draws heat anyway.
Heat doesn't rise as much as it transfers to colder objects. The heatsink is lower than the heat source (GPU, power phases, RAM chips, etc) but it is also colder so it takes the heat anyway.
Heat rises in fluids such as water or air because it increases the pressure, lowering the density and increasing velocity of the fluid's particles. Lower density fluids rise above higher density fluids. A good example of higher densities rising would be pouring vegetable oil and water in a measuring cup.
Point is that the fan being put above the card wouldn't help much, if at all.