Well, my favourite way to attach heatsinks to the back of vid cards (specifically my V3-2000 @ 187) Is to cover the back of the card in electrical tape, except the area with a square high concentration of contacts directly behind the vid.chip. This area is then liberally plastered in NON-CONDUCTIVE thermal paste - the generic white stuff works fine here. The heatsink is then glued to the electrical tape. This prevents the card being unrecoverably modified in any way, but provides good heat transfer...The best reason to do this is if you own a vid card which is not memory-bandwidth limited - mostly older 3D cards - with the newer NVidia Geforce2's, there is hardly any point in overclocking the GPU.
I only put a heatsink on the back of my Voodoo3 because I needed the slot below the AGP slot, and therefore only the stock heatsink would fit on the frontside of the card, and that is hardly the most impressive heatsink in the world..
cheers
tweakr