So after doing some research on this forum and other sites I've decided I'm going to get a PCI slot exhaust fan for my GPU but I'm wondering where would be the most effective spot for it; Below the GPU (though I'm thinking that may starve the GPU's intake fan of air) or above the GPU (because that layer of green metal on which the GPU sits isn't that thick and also since heat rises I'm pretty certain that this would at least take heat away from the top of the GPU)?
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.
Well it all depends on your graphics card , is it a reference card?
If its a non reference card they usually blow out hot air into the case and therefore the exhaust blower slot fans can work well directing the hot air out of your case and would be best placed under the card.
If its a reference card then you will be better off getting a slot card that just blows air onto the card.
These are great slot coolers - AzenX BT-SC70BBL Blitztorm PCI Slot Cooler, but again it depends on what card you have.